As a man who is in the "high 300's" himself, I have successfully climbed the steps numerous times with no problems, and I use the bathroom with no problem, except for the occasional charlie horse (I'm 6'3", so I'm cursed with being big AND tall). It's a challenge, but one that you just had to adapt and overcome. I hope he gets the opportunity to go with you.
As far as having a talk with him about losing weight, save it. We know the ramifications of being big. If it were so easy, we'd all be skinny. We just like to give the perfect people something to talk about. ;)