Pets

It could be parasites such as fleas and such. Did you make sure he did not catch any ? Its possible even for indoor cats to get some.
Here is what I found:

Excessive licking of the lower belly fur in cats can be caused by both physical and psychological issues. It might be that your cat is struggling with parasites, allergies, or dry skin. Other reasons can be that your cat is bored or is facing stress and anxiety. However, contrary to popular opinion, licking off entire patches of fur isn’t typically related to a behavioral problem in felines, and it’s usually a sign of an underlying medical condition, such as parasites or a food allergy.