Two suggestions. If your boy wants his own board, the Trick Boardz freestyle is (almost) neutrally bouyant in the water, he can actually position the board such that he doesn't need to flip it up. I love the freestyle for just this purpose.
Another option that I have found useful is a two stage start - we call it "faster slower"
When your boy is in the water, engage forward and very VERY slowly edge forward. At the same time have your boy scrunch his legs up with the movement. Such that he's in a tucked position. When all the slack of his "legs" is out he will push down with all his might on his heels. The slight movement forward will assist with flipping the board up. Then YOU dad, check in the mirror, when you see the boards leading edge out of the water, give it more gas.
The little ones literaly POP up.
Keep us posted on how it goes.