Sorry to hear about the problems you're having. It's a situation I see on a daily basis (I work as a McKenzie Friend - a legal assistant for cases like yours) and I am a volunteer with Families Need Fathers.
I would strongly advise you contact them and find your local branch. You will get emotional and practical support. Often there are pro bono solicitors there who are able to give you legal advice.
But what is worth considering...
Document what is happening. Keep a diary. Keep records of emails, texts, letters and think about recording handovers if they are abusive and upsetting for your son (it is legal to do so without telling her but they may be useful down the line). Make sure you have a record of when contact takes place.
In an ideal world I would say that if you feel your son is in danger that you don't return him to mum.
From my experience however the likely outcome of that however is:
a) a knock on the door from the police who (without legal basis) demand you return him or threaten you to arrest you for a breach of the peace*.
b) when you do return him she refuses to allow him to see you again compelling you to take the matter to court and his time with you is limited.
c) you are contacted by social services demanding you return him.
So this is a high risk strategy for you are a father. And with reduced or no time with your son your capacity to protect him is further diminished.
In this case think about initiating mediation if you feel that in the short term is safety is at risk. Specifically - organising it yourself. Google `National Family Mediation' and organise it...don't ask her if she will go. Statistically there is a 20% chance of it working, but there is a reason for doing this.
Firstly, there is a chance it may work and it is worth trying because if it comes to court, things may well get worse before they get better. Secondly, you will have to have attempted mediation if you DO go to court.
Hope this helps.
I would strongly advise you contact FNF (link above) - and the same goes for Mums as they face most of the same problems dads do. If you want a chat you can call the helpline - but you're more than welcome to get in touch with me for some pointers. It's far from as bleak as you may think it is and there is a lot you can do to help your son.
Stay positive and get in touch if you need a chat.