Bf (27M) is sad because he can’t go to the gym, how do I (24F) support him?

Context: My boyfriend (27M) and I (24F) love each other, we’ve been together for 5 months and pretty much everything’s fine. He’s supportive in everything I do, makes me laugh, and overall is a walking green flag. There’s a problem, though.
Some months ago he got in the hospital because of miocarditis. He’s fine now, but still has to take medicines everyday, for at least two months more (then the doctors will declare if he’s definitely healed).

Now, here’s the issue I don’t know how to properly act about. Before the hospital, he’d been hitting the gym for two years, and would be ecstatic about it and the results he got. Of course, he can’t do anything at the moment in order not to strain the heart, maybe just some strolling after work. And, well, we’ve gained some weight, especially after visiting his parents on vacation and eating lots of delicious homemade food. Nothing tragic, really, it’s maybe, I don’t know, 2/3kg more? Gained in 4 months?
Now, I’ve had some struggles with my body in the past, but I’ve quite grown out of it – I would like to start going to the gym too, though, when bf is getting better. Him, on the other hand…

I can see he’s frustrated, though he constantly “jokes” saying that he’s too fat, that he sucks, etc. Mind you, he only says these things about himself, he never said anything negative about me or my body. And when I tried telling him that well, I’ve gained weight too, he said it doesn’t really count because I have boobs.

I know that he tries laughing about it, but I can always sense some sadness when he does. I’ve tried reassuring him saying that there’s only two months of therapy left, but he reminded me that we can’t be sure about that (there have been cases of people who had to keep on living like this for two/three months more). Soooo what could I do to be more supportive? I love him so much (and make sure he knows it everyday!) and I still think he’s the hottest man I’ve ever seen. Anyone who’s been in a similar situation?

TLDR: After going to the gym for 2 years bf can’t do it anymore because of health issues. Gained some weight but can’t go back exercising yet, how can I make him less sad about all of this in the meantime?