I agree with the chorus of responses commending your husband for holding up his end of being a parent in spite of his son’s rejection. It stinks to be in that position, but punishing his son by cutting him off will burn all bridges, and there’s a pretty good argument that some obligation exists (although the extent of the obligation could certainly be debated) irrespective of gratitude or lack thereof.
I don’t think that financial support has to be completely without strings, though. I wouldn’t suggest using it to try to force personal interactions that the son doesn’t want, but if I were paying for college, for a kid that wasn’t communicating with me, I’d be insisting they sign the paperwork to give me portal access, so that I could see his grades and be certain he was on track. Also, “paying for college” doesn’t have to mean a blank check - it would be entirely fair to stipulate a budget (the full cost of attending his in-state flagship, for example) and decline to go above that amount.
But, the “I paid for my own college” argument, from people in our generation, really doesn’t wash. The cost of college has risen far faster than inflation. What was possible when we were in college really isn’t, anymore.
It doesn’t sound as if the rift in the relationship has been going on for all that long. This happened when he was 15, and he’s, what, 16-17 now? It’s hurtful that he’s withdrawn from the relationship, but retaliating financially would be petty and counterproductive, IMHO. Being the bigger person is a big part of parenting. And advocating against that could put your husband in a very uncomfortable position and damage your relationship.