Hi Nathan
You poor lamb. And how sad for your son.
Don't give up.
My husband and I split in December. It's been v v messy and traumatic for our children. So, my duaghter, 15 initially saw him of her own volition, then saw him because I made her, and since July, has had no contact with him whatsoever. He was very badly behaved in front of her and she simply had enough. So she wouldn't take his texts, hides when he comes to the house to collect her brothers, won't talk to him and wants to totally disconnect her life from him. Initially he texted every day - crappy things to be honest that made her cringe. Then he went to a text a week. Then nothing. In August she got A*'s for her GCSE's. I emailed him to let him know and he told me how fab she was. But he didn't text her or tell her she was fab. When he is at the house he never asks for her or where she is. He makes a great show of contacting her tutor at school and reading up on 'A' level stuff. This is a show of what a great parent he is. But he's missed the point. He's not sufficiently bothered to text her every week/day; to send her a card to say 'well done' or a teddy that he saw which made him think of her. She is hurt because he has given up. I know this is what she wants, but - she's a female - contrary goes with the territory!
He should stick at it and be ignored - no matter how hurtful, rather than fade to zero in the background. Even being cross about a text is a connection between two people. No text, no connection. Some day she'll get a grip on her anger and be inclined to see him. But only if he has been constantly there. Out of sight (and text) out of mind. Every day she grows a little further away from him and I can't help because he won't let me.
I'm telling you this because I'm witnessing the other side and how it feels for my daughter.
Your ex is clearly engaging in parent alienation - loads on the web about it - I know because I've been accused of it - v v unfairly. It's a hard one to tackle, but just keep going.
Every so often drop them a note. Letters are more real than texts. You don't need to bribe with gifts. Draw a picture of a tree for each child and ask them to put leaves or flowers on it for you. Send a picture they can colour in. Send a badge, a fluffy toy, a shell from the beach. Use pictures because of the age - you'd don't want to have to rely on the ex to read a letter to them. Say to them when you see them next I am going to send you the biggest leaf I can find and you have to find one that's bigger and if you do I'll give you a sweet when I see you next. Then stick a leaf in an envelope.
However you do it - whatever you do - use every channel to them that's available if you can't see them. Ignore the rubbish about not wanting to see you. My son used to cry like a banshee when I dropped him off at nursery, but howled when I picked him up. He always had a ball.
Be constant, be calm, be loving, don't lower your dignity or do anything that you would not be proud for your son to know when he is older.
Don't give up - you might actually have a better time thn you think using the Royal Mail. You'll be excited anticipating them getting the post and they'll start to look forward to what comes next. You'll be thinking of what to send them next. They will remember this. It's special.
Good luck
xx