How do you think he is likely to feel about it? I know you've said he can be defensive, but is he the type to be embarrassed that it's noticeable, or annoyed that you have mentioned it?
If he's likely to be embarrassed at you noticing, he would probably be mortified to think that others will be aware of it too - maybe not so much in lockdown, but when it's over and he's mixing in company again, so that could be a way in for you. You could gently point out that there is a problem, and offer to help him to get to the, er, bottom of it.
If he is likely to be annoyed, you could have a word with his GP and ask if they would bring the conversation round to it at his next check up. I'm sure they are used to having to broach embarrassing subjects with reluctant patients, and can do it tactfully.
Either way, I would be gentle with him. Some people are very embarrassed about this sort of thing, and as he can't help it there is no need to make things worse (not that I'm suggesting you would )