poor little man! My boys have had viruses and stomach upsets before without having a temp so dont rule that out. I would have thought if the food disagreed with him , esp as it was fish he would get some kind of stomach cramps and be uncomfortable, doesnt sound like he has these by what you said.
With most stomach bugs the sickness should stop within 24-48 hours so keep an eye on him. Its a good idea to nip him to the doctors tomorrow if hes still no better.
Whenever my children are ill I make a bed for them on the sofa and just let them chill, if he wants to play than thats cool. I wouldnt worry about giving him food unless he really wants it and just keep it to drier things like toast and biscuits!
Just keep up the fluids, make him drink plenty and keep an eye to make sure hes doing wee's, if hes not get him to the doctor sooner as he might be dehydrating. Oh and my doctor told me never to give a child dairy products when they were sick because the bacteria in them can make the stomach worse!!
I know you probably knew most of this but thought it might help!!

heres hoping he feels better soon!