I agree with the LF Nemesis. It has been my sons board since he was about 40 lbs.
When he learns, the speeds will be so slow (under 10 mph), that a helmet doesnt seem terribly useful.
I would say that the easiest way to teach him is to get up on a boom if possible. This keeps him right close to the boat where you can coach him.
This is my son on a short rope connected to a boom.