Hi
FM 70 has pointed towards what I was going to say 🙂
Basically, you have been working under the idea thaty you are temporarily caring for your son, but will at some point be sending him back to your ex to live, at which point you will want contact (and have to make arrangements for this with a 200 mile gap between you and him).
The simple fact is that you have residency at the moment. You priority now is to get your son settled into a routine, have proper living space (hopefully he already has), regular friends etc, both for his sake and because the more settled he is, the more likely a court is going to order that he stays with you if your ex tries to go for residency. It's also slightly unorthodox, if your ex does try for residency, the longer the process takes, the more settled your son is going to be, and as I said, the more likely the outcome that the court awards residency to you, so while you have to comply with any court orders, you can drag out the process as much as possible.
The first thing I would suggest doing is nothing - ie, don't offer to hand your son back, don't apply for any orders, don't offer mediation, do nothing until you have to. The basic idea is that the longer you can delay the start of the process, the more your son is settled with you. While I appreciate that you want your son to have contact with his mum, and FM70 is completely correct that contact is the right of the child, you first priority is to have residency of your son so that you can look after his wellbeing in the long term.
Now, while you are doing nothing, you should also be gathering as much information as possible about what the procedures are for gaining a residence order, so that when, and if your ex starts to want your son, then you know what procedures you need to start, the first probably being to suggest mediation for your ex to have contact (and I stress, contact, not residency).
With regards to your point about a psychiatric assessment for your ex, it's something I would be asking for during proceedings - to be honest, if you have concerns for your son's safety or wellbeing, at the point where you are discussing contact, I would seriously consider suggesting that contact is initially supervised - that also applies if you think that your ex may take your son and not return him.
A very good starting point is yoji's guides to representing yourself (and hopefully yoji and others will pop on here and add more information, or alternatives) at the top of the legal sections, and assuming you don't currently have a solicitor, then you can call the Children's Legal Centre for free legal advice, but keep coming on here and we'll help and encourage all you can.