Be careful with things like yoghurt, ice cream, milk etc. It can coat the tonsils and add to the infection. I used to get chronic tonsilitis as a teenager before having them taking out and was always advised against dairy. For something soothing you're better off giving him icy poles. Agree with BG's comment that the sooner he can eat normal food the better, even though the poor little guy won't feel like anything for a while.

Hope he's feeling better soon.