How long have you had him for? When I got mine at around 5 inches he didn't eat for 3 weeks.
There are couple different things to try as well such as using fishing line attached to the food and letting it hang in the aquarium so the fish can always see it when swimming. I used chopsticks and eventually got my guy to take the silversides I was trying to feed.
Another thing to keep in mind is that, although they will eat anything before they starve themselves to death, is that the fish might not like prawns. Some fish like other things so it is good to try tilapia fillets as they are cheaper than prawns, silversides, smelt, minnows, or anything else you might want to get the piranha on.
Last thing to try is something called garlic guard by seachem. It smells like a lot of garlic and is supposed to mimic some type of algae or something that makes picky fish eat. This is what I used to get my guy over to pellets.
If he is new as well to your environment he might not want to eat in front of you, so try the fishing line trick and leave the room. I wouldn't leave food in for 2 hours, but if you put it in for 15 minutes and leave with lights low then he will either eat it or not. If not try it again next day until he eats it, as they will not starve to death if all other things like water params are in check
