The reason I mentioned remarriage is you can't claim ancillary relief to tie up the finances if you have remarried, but that doesn't apply here.
I think when the child reaches 18 he could ask the court to order the sale of the house pointing out the child no longer dependent and his ex has made no endeavour to release him from the mortgage.
Another possibility is to allow the house to be repossessed.
A third option might be to proceed under the Trusts of Land and Appointment of Trustees Act 1996 to secure ownership for the amount currently mortgaged. If someone makes a financial contribution to a property they are entitled to make a claim to a share even if their name isn't on the deeds. This sometimes happens with cohabitants.
Rest assured it will come out in the wash, but he will need to consult a sol (unless he lets the house be repossessed) and that might cost a few bob.
Thinking about this a bit more before rushing off to sol he needs to check the terms of his settlement. In court orders that transfer the property and leave a joint mortgage, there are usually terms which require the sole owner to indemnify the other party against any liability that arises out of mortgage defaulting, and that would be enforceable against the property.