Well, you created the burden by saying you would pay his loans while he is in school. BUT while he is in school, his likely doesn’t have to make payments on his undergrad loans — though interest will accrue. Read this article for more.
The idea of law school is that he will graduate and earn enough to pay back his student loans! Maybe the smart idea is for you to help him with his tuition –and let him deal with his loans when he graduates, and payments are due. There may be some helpful programs at that time.