You can ask at the chemist for something called Kenalog (sp?) that is like a super quick dry cement to put over mouth ulcers. I haven't tried it, but might be worth a shot. As for food, I would imagine cold/soft/liquid might feel best, but I'd just go with whatever he is willing to eat for the time being.

I can't remember what I used on DD's nappy area, but maybe ask at the chemist if something in particular would help, and make nappy changes more frequent.