number one, from my POV. I am Obese. The last thing I want to do is be put on display by being seated in Handicap seating. Its like wearing a circus tent to a funeral. People stare, and staring people judge. When someone is so obese and cant walk, it causes staring. Then the mind games begin.
Number two, they do hurt most likely. Having to sit in a cramped, uncomfortable chair for an extended period of time HURTS... if it DON'T fit don't wear it. I doubt your FIL WANTS to be fat, made fun of , ridiculed, OR screamed at for not coming cause he is fat. Being reminded on a daily basis is not helpful. HE KNOWS he is fat. He doesn't have to be told. He doesn't want to be catered too and he defiantly does not want to be berated.
Just talking to him in a personal manner one on one, with no one else in the room may help. Saying things like, I want you to be more healthy, and enjoy your grand kids lives. My mom was also a very large woman. She was devastated she could not run and play with her grand kids, or go to amusement parks or beaches. She explained why she couldnt go and the grand kids understood for the most part. Yes it made them sad, but eventually they were just ok with saying. "Ah thats something my Grandma cant do"
If you dont like to watch fat friend's kids in the pool, just be honest, not mean. Say you cant watch her kids properly and take care of her own. She can swim right? shes just fat. If she can swim she can get in and save her kids. If she is laying around being lazy, that has nothing to do with being fat. She can go in the water with CLOTHES on. My god when did bathing in a private setting mean you have to wear dental floss and a lace doily? Tell her to get up and take care of her own children, bring clothes she can get wet in. If she cant do that . DONT invite her unless you want to be caring for the kids.
not trying to make it sound like bashing, but I am just trying to help you see why some may not do what you like.
Change yourself to be more understanding, will be hard. If you cant already understand them, then its time to look at it from another vantage point.
I am very heavy, but I do everything physically I can with my kids. I do go swimming, I go to things, I just dont do what is weight restrictive.
For what its worth, I come from a mixed family. My mom was always heavy. Even at her skinniest before her kids, she was teased and put down. She was BEAUTIFUL. Gorgeous features, but always heavy. My dad was pin thin. Still is, save for a baby gut from old age. He was always teased for being too thin, and then ridiculed for marrying my mom. I am one of 4 kids. My oldest sister and younger brother are model thin. They did everything to gain weight. Oldest sister was thought to be anorexic by her school and called my parents of several occasions. She ate all the time. Never puked but was super athletic. She would run 10 miles a day. Me? I was never overweight till senior in high school when my metabolism took a hit. Didnt know it till later that I could have fixed that earlier. I am very mobile, probably 75lbs over what I should be. I am very tall and built very solid so I dont look it, but I do look fat..
My second sister, is morbidly (scarily) obese. She is 40, and cant get out of a chair with out help. Last year when we invited her to go to my daughters birthday at the zoo, we had to rent her a motor scooter, just for her to get from the parking lot to the play area. It was so embarrassing for her she cried the whole way there. I apologized a thousand times and decided if it was in public I would have something separate for them if they didn't want to do it. I will always ask her first from now on.