You can get numbing gel from the chemist or local women’s and children’s hospital. Try putting it on just before eating and see your dr if it is a regular problem -

The fruit will probably hurt him due to the acid content. I would try a very bland diet, consisting of things that are easy to swallow and chew, like mashes etc. If an ulcer goes beyond a week it is advisable to see a GP, particularly if it has been recurrent

As long as you’re treating the ulcer, any food, his normal diet should be suffice. It’s hard to imagine an ulcer would cause this much distress. Or you could try dairy products, yoghurt, custards etc should be kind to a sore mouth

Mashed and or pureed foods are a good option.

My niece has potatoes, pumpkin generally mashed. Carrots broccoli. She finds most veggies don’t hurt. Plain boring steamed veg. Also grilled fish seems to be ok also. On my daughter I used a mouth gel to numb the area. I think it was a diflam brand mouth ulcer gel. Bonjela also works too. Stings at first but will eat anything after that.

mashed potato, soup, hard boiled eggs, mashed eggs, scrambled eggs

I would go with mashed and or pureed foods until the pain leaves, try a little mouth gel such as bonjela (its not just for teething I use it too!). I hope he gets better soon!