My apologies - I missed your reply
There are two legal issues you need to be aware of.
The first is that without a Declaration of Trust (which your conveyancer should have advised you about) then technically your ex is entitled to half of the equity.
However should the matter go to court the substantial difference in contributions is likely to mean that the court will say that your deposit needs to be returned before the balance is divided between you - if not then you may have a claim against the conveyancers!
In addition under Schedule 1 of the Children Act your ex is under a duty to help you house the child until the child is 18 - and if the house was sold you would need all the equity to rehouse yourself so he would still have to wait for his share
Unfortunately, there is no law that says he has to co-operate with switching the mortgage from a repayment to an interest only one - and I suspect that the lender will not agree without his consent.
Your only option is to inform him that if he wishes to avoid mortgage arrears threatening his credit rating then he either agrees to the switch - or pays the whole of the difference
I hope that this is of assistance - please ask if you need further details