HELP...how can I protect my son!?

You could start by talking to his school and finding out how he is there.  Tell them your concerns and what has happened so they know to look out for changes in his behaviour.  Ask them if they consider he has ADHD and they can make a referral. You can also make your own referral to social services if you believe he is being harmed in any way.  It won't go down well with your ex but your child's needs are the priority.  Do you have an order in place for contact or have arrangements been made between you?

@Champagne Thank you for your reply. She abruptly stopped all contact around 4 weeks ago after having continuous routine contact for the last 6 years through mutual agreement with our lawyers. My lawyer has started the ball rolling on a court order so this can’t happen again (this is the 3rd occasion throughout his life that contact has been shut down) but I’m aware this will take time.
However he now has 2 young siblings here who are also being emotionally affected by him no longer being around.
As a father I feel I cannot just allow my 3 kids mental /emotional well-being to be potentially affected so unnecessarily. 
Regarding school, they go back on Monday so speaking to the teacher is a possibility regarding ADHD...although it’s that conversation with his mother that has landed me where I am unfortunately. Not sure how that would affect things or make thing worse for him at home. 

@derek8626 Good plan to speak to the school.  If they have concerns about ADHD or how he presents in school, they will discuss with mother.  They will be used to dealing with these situations and mother will react differently if it comes from the school.  If the school has no concerns then you should feel reassured.  Good luck

@derek8626 Hello. Whilst I cannot offer you direct advice into your current situation, I would suggest that keeping a positive relationship with the school is to be encouraged. Even though you are unable to see your son at the moment, as his Dad you are still able to take an interest in his schooling and how he is managing. Could your sons younger siblings make something for him that you could send to his Mums home? Can you do the same, keep in touch with him this way - reassuring that you still love him and that you look forward to when you can spend some time with him again, perhaps suggesting what activities you could do? I realise that the post could be seen first by his mum, but at least you are attempting to keep contact up with him, and also including his younger siblings too. Now that the situation is in the hands of the Courts, I understand it must be frustrating for you having to wait, but please don't give up. When you are talking to your other children, make sure too that you include your son in that - so they know you love him as much as you love them. Please make sure that you look after yourself too, remember to talk to those who are close to you, or if you prefer, seek some counselling time to talk through your feelings. It's not a sign of weakness, but a sign of strength. Take one day at a time. Perhaps try to contact your Ex-partner which ever way you do normally, about setting up a video call with your son. Wish you all the very best, Fegans PSV.

@clarinet Thank you for your reply! It has given me some food for thought. 
I was reluctant to phone the school as I am not familiar with the teachers yet...this being the 3rd primary school his mother has moved him to. But I definitely will be doing this. 

Unfortunately mum is not the easiest to deal with and it would seem she would rather this other side of his family didn’t exist. We have literally zero contact. All calls ignored. All txts delivered but no reply and now lawyers letters being ignored! It just seems very unfair for my son and his siblings to suffer. I have expressed my love for him and how much we all are missing him in txts...I can only hope he has read them. 

My main worry is how my son will be when he eventually comes back to us. I have no control over the lies his mum could be telling him and the manipulation of his feelings towards myself to damage the strong relationship we have(or had). 

I know it’s a waiting game for the court order to proceed, but this void between now and then is just unbearable. But like I assured him - I am not going anywhere and I won’t stop until he’s back with us again. 

@derek8626 - thanks for the reply. It must be so frustrating for you at the moment. Hold onto what you do have control over in your life - i.e your current family life, work etc, and perhaps have a think about something special you could make for your son, so when you next see him it will be something that you can give him to show him you were all still thinking of him, or another idea if you have one. As far as school is concerned, as his Dad you are able to have a good relationship with the school as a father interested in his sons progress. Let us know how things go in the up and coming months. Please don't forget to talk with someone if you feel it's all getting too much for you. 

Kind Regards, Fegans PSV

@derek8626 Youre his dad. You are strong. You must be strong for him and your family. This will resolve. Its hard but be patient, be encouraged and dont panic. Try and let go of what is beyond your control. Your sons love for you will not instantly fade. he probably feels as affected as you have been. be strong. Its what you need to focus on now. Im sorry his mother not only doesnt care about you, but clearly she doesnt truly have his best interests in mind.

Okay, so this is serious! Although I may not be the best person to give you advice, my suggestion is to look for alternative ways to connect with your son. Talk to his friends, reach out to his school, or speak with his teachers. They can make him understand your feelings for him. If this doesn’t work, look for social services and ask if there is a way to contact your son.