Perhaps the better intermediate step might be to add a longer seatpost and stem for now. Get him by until he truly needs the larger frame. My oldest son turns 8 this October and is 4' 2" and still fits great on his 20" wheel bike. It's a good enough bike that I plan to adjust it to fit for as long as I can, at least until his younger brother can still using it so another 2-3 years hopefully.