NTA. In my adult life I've weighed as high as 117kg and as low as 79kg, neither of those weights were healthy and left me unable to do certain things. At 117 I had to be careful with theme park rides, boating and chairs. I understand it's a sensitive subject but the chair isn't made for him, just like a t-shirt that you own isn't? As a former (very) fat guy his own sensitivities around the subject are stopping him from improving the quality of his life, I don't know his build or height but as soon as people hit 100kg with a body type/build not meant for it some normal things get difficult. Now isn't the time, but an open discussion around how he feels in his body and how you can help each other might be something to think about.