I'm glad to hear you will be helping your son but I think you are perhaps missing the point. We all need to work within the law as it exists rather than invent a system we wish existed, but doesn't. In the Matrimonial Causes Act 1973 a "child" isn't defined and under s29(1) of the Act the court cannot order financial provision for a child who has attained the age of 18 unless;
"(a) the child is, or will be, or if an order were made without complying with either or both of those provisions would be, receiving instruction at an educational establishment or undergoing training for a trade, profession or vocation, whether or not he is also, or will also be, in gainful employment; or
(b) there are special circumstances which justify the making of an order without complying with either or both of those provisions."
So your ex can make an application and the court has the authority to grant maintenance for the benefit of a 'child' over 18. (Whether the court will award maintenance or not and how much depends on the individual circumstances.)
Please don't shoot the messenger!
