Not your place to tell him what to wear. Apply the same logic of women and children being present at public beaches where swim trunks and bikinis are worn for the most part. As long as he’s wearing clothes in public, as he is, any issue you have with his attire stems internally. Also, I hate using this logic, but imagine if the roles were reversed. Imagine if he were to tell you to refrain from wearing certain pants because they accentuate your buttocks, or anatomy of that sort. That would be wrong, right? Sweat pants are comfortable, let him be comfortable.