I am a lawyer in Scotland. What is the basis on which you contend that your husband has a duty of maintenance?
I don't think you can easily put a formula forward for this although I do understand that a CSA type calculation might be a starting point. If you have a look at section 1 of the Family Law (Scotland) Act 1985, however, that provides for a general duty of maintenance for a child of between 18 and 25 who is in full time education or training. He isn't of course but a "needs and resources" test would probably be appropriate. In other words what does your husband's son need where there is probably state assistance and of course a joint duty of maintenance on the part of both parents. And of course what is affordable given your husband's financial commitments. I would find 30% of income unaffordable and inappropriate. About 15% would be my subjective guess.
Yes it would.