I sympathise, I had a 2 year old and 4 year old really into the tanks, sticky fingers on the glass, wanting to feed them etc. I solved the problem with educating them that they were harming the fish. I promised them if they left my fish tanks alone they could have their own in the bedroom. It worked great. If you are worried you could get him some robo fish to start off with. Thankfully mine are older now.