Dont take offence to this anyone, but tell him to tommy tank in the mayo(or just say he did) and tell the lad when he notices its been used. Go up in abit of a shock asking if he has had any because some was put in there with human product in for science/biology but he didnt think to mark it up.
Or, just put a note on saying that if anyone touches the food they better have some nice stuff to take in swapsies.
If it was me and i thought i could take him, id have him against a wall. My food is my food. If i dont offer it you keep your hands off. Or i'll take your food or whatever else you have i might like. Tell your son to take the lads pillow, hide it 'for a laugh' then take another pillow 'for a laugh' then if he doesnt stop, the mattress goes somewhere.
After all, he takes food and probably thinks its funny, so take his sleeping stuff.
Never been to uni though, at work theres a certain respect for peoples things. You dont touch other peoples food and only touch the milk in the fridge after asking someone if you can borrow some.