Yes, he would be a Son-In-Law. The spouse of your kid is your [son/daughter] in law.
Exactly he would be a son-in-law. There is always confusion when trying to figure out how your husband fits into the family tree, her parents, etc. I worked on several college presentations and papers on sites and sorted essays by categories with ancestors' trees in them https://studyhippo.com/essays-by-categories/. So to avoid that kind of confusion, I recommend you write down all your thoughts on the tree, and after you are done with that, sort all things one by one. And If you need to add something, add as you will discover it.
