Skireel that is sweet!
My son started riding this year and is doing great. He is on his own now on start ups and loves his board. He is 6 now so a little older than your boy however the Motive 119 that (slipknot)is talking about is a great bord to start him on. The problem that you are going to have however is getting him to be able to flip the board back over if he goes over the top of the board. What I mean by that is sometimes when I pull my son out i give a little to hard of a pull and he comes over the board and is laying on his stomach in the water. When u get a little one in this position it is difficult for them to get the borad back over to their back starting position. I started my son on the shoreline and taught him how to grab the board to help him flip it in the water before he went out.
Now he is riding for about 30 min runs it is great.
Good luck and keep em boarding.