I can't see a problem with the sashimi as its just rice, veg and seaweed roll. Could sushi be a bit hard to chew? Not sure about it being uncooked either. Hoe do japanese people feed it to their youngsters? Any Japanese members?
It's probably too late to comment now, but just make sure it's not raw tuna. The only time I've ever had a severe allergic reaction was after eating fresh tuna steak - real heart-pounding stuff within 15 minutes - blood-shot eyes/double vision, top-to-toe rash and breathing problems. Apparently it's a common reaction to the toxins of bacteria in either raw or fresh tuna that's off.
Sorry, I sound like the voice of doom but it was pretty frightening at the time!
I took my two to Yo-Sushi when they were 3.5y and 14m. I didn't give them anything raw (stuffed my own face though ) on the principle that if we shouldn't give them lightly cooked eggs, then raw fish might be just as potentially dangerous. Nonetheless there was lots of cooked stuff that they enjoyed.
Don't be in a hurry sophable, there's plenty of time to widen his tastes. In any case, liking something today means nothing for the future. Ds used to love peas, suddenly decided he didn't like them and now genuinely gags on them - but only if he knows he's eating them.