if he's just moving it to under the wing, that's fine, gas engines have pumper carbs that don't care about the tank position, they'll pull fuel about 4 feet vertically I believe, plus under the CG there's less change in balance when flying anyway.

on my ultrastik lite I moved the fuel tank to under the wing and moved the battery to under the back of the wing to get it balanced, haven't flown the dang thing yet though, finished it right as the weather turned crappy and the daylight went away quick...