There's nothing you can do to stop your ex seeing other people. That is her right, but you can certainly use the courts to protect the level of contact you have with your son.
A couple of questions; do you pay maintanence for your child and how long has it been seen you split with your ex?
As I said earlier, as you appear on the birth certificate you have Parental Responsibility and that does give you a number of rights. These rights include being consulted about your sons choice of religion, education, etc. However PR doesn't give you the right to contact, the law see's that as the right of a child and that right is controlled by the Resident Parent (in this case you ex). If the Non-Resident parent (you) isn't being allowed contact or has had the level of contact reduced, and the Resident Parent continues to be uncooperative, you will need a court to make a defined contact order.
I would expect a court to restore the level of contact that you had enjoyed previously. It does appear that you actually had a shared residency arrangement with your ex, as your son was spending nearly half the week with you. This essentially means that your son had two homes; one being yours and the other being his mothers. These arrangements have become quite common in recent years.
So, what to do next?
Have you got any evidence of your previous agreement? Letters, emails, texts? If so, keep them and ensure that you keep a record of all future communication.
You need to write to your ex (by letter or email) and let her know that you're unhappy with the reduction in contact. Explain that you've taken some advice and you understand that you have PR, but if she's unwilling to increase contact you will have no choice other than to look at involving the courts. Explain that you've always wanted to be able to address the matter of contact as amicably as possible and that, if she thinks it's a good idea, you can both go to a mediation service and work out an agreement. A court will have expected you to have tried mediation anyway, so it's best to offer it to her in writing.
If she doesn't go for this then you need to apply for a contact or shared residency order.
You don't necessarily need a solicitor to do this, as many Dad's are now representing themselves in court and you'll notice that Yoji (one of the other Dad's) has posted a really comprehensive guide to reperesenting yourself at the top of this forum.
I would write to her and then give her 14 days to respond. If she doesn't reply, then make an application to the court. It will take 6-8 weeks before you have your inital hearing, but you may find that your ex restricts contact even further during this time. Again keep a record of these occasions and present it to the court.
If you need clarity on anything I have said, just get in touch and I, or one of the other dad's, will be able to help you!
Cheers,
FM '70