I’m about a size 22-24, all belly and wobbly thighs. When I bounce, everything jiggles. My parter wears a size small, and he’s not broken or hurt. He has me lie on him too, and can still breathe.
Think about this logically, your partner is with you, he obviously knows what your body looks like and he’s not done a runner. You’re doing both or you a disservice by hiding yourself and holding back.
Start with baby steps of kneeling on pillows beside him, I can’t kneel too long either, bc of scars on my knees. I’m tempted to get knee pads to see if that takes some of the pain away. I can make them sexy 
Does he want to try any other positions? Even just a light change to positions can give you different sensations. Have a look at this thread PotW: The Massive One. There’s so much more to sex than missionary and bjs. I love missionary for the eye contact, its my favourite position to end on, but theres a lot of fun to be had. It sounds like its going to take you to say, lets try… I hope he is secretly adventurous.