If he has the money, then there is no reason for you to release funds, especially since there does not seem to be a specific agreement that your funds were to be the contingent measure if and when he defaults. It also does not seem to be the case that he would suffer from release of funds to your son even under present medical circumstances, as funds have already been stated "earmarked" for your son's college fees.

If he insists on your contribution, for whatever conditions he is compelled to impose on you and your son, it might help to present your own set of conditions i.e. some form of collateral such as a shared equity on the house (provided it has a clean title, of course) in proportion to the sum you contribute plus interest. In that way, you would secure your retirement funds. You could perhaps consult a property lawyer over there in relation to tenancy in common or a counterpart of the sort.