You could get one of those shock mats that teach cats to stay off the counters.
Just kidding. I would go with the suggestion of tell the kid that anything he puts in the tank may be poison and kill the fish. As he gets older, and questions that, tell him that the toys may be dirty or he may not have washed his hands throughly so there is soap that may get into the water....all of which could kill fish.
Or maybe borrow one HUGE fish and have it bite his finger once. I'd bet he never sticks his hand in again. Just kidding on that one too! =)
Is the tank low enough he can reach it with out standing on something? Perhaps raise it higher.
Maybe find a cracked tank or something and take it outside, fill it with water/gravel/fake plants and let him play with, hoping he will get it out of his system. There could be an inside tank and an outside tank, with the inside tank being off limits.
You could keep him out of the room with the fish tank unless supervised. Not practical if tank is in central living space.
If he was a cat, I'd squirt him with water every time he touched it. =)
Again, I'd go with the poison route.....