Your teen is leaving food under the bed (also pizza is a meal, not a snack) so obviously he can't be trusted to have food in his room. That's just common sense, surely?
Hmm. OR we could just consider that teenagers are not perfect robots and will do things they regret, things they shouldn't have and make mistakes, just like everyone.
Finding a piece of pizza under the bed and jumping to outright ban and a 'you cannot be trusted' approach is overkill and unlikely to have a positive outcome.
I'd have rolled my eyes, called ds and told him to be more careful then made him give his room a good clean...and warned that if this was repeated we may need to think about banning food in rooms.
Both my older ds's are allowed to eat in their rooms. They have an end of day cleaning rule though. Every single day before bed they have to empty their room of rubbish, plates/cups and any dirty laundry, putting it all where it should go. It takes 5 minutes a day but keeps it presentable ish.
About once a week they clean it themselves, including changing beds, dusting, cleaning TV and gaming monitors and hoover or sweep and mop (one has wood flooring, one carpet). About once every 3 months I spend a couple of hours with them and help them do a big 'rip apart' clean and sort out. Inside drawers and cupboards, around edges of furniture where an accumulation of 'stuff' lands, clean the window and skirts etc. Other than my 2hours x 4 times a year though they manage it themselves.