If you plan to paddle any distance with him paddle size matters a lot. The extra effort required for a large blade is hard on young kids and old men. I would probably reduce blade size about 50% and paddle a little slower also. When my kids were young I was able to find some commercial paddles the right size, I can't remember the brand. Take plenty of stops and let them explore and they will like it a lot more.

I'll second what Scott said. It's important that children have paddles sized for their bodies. Paddles that are too big will be unwieldy and they will tire quickly. Unfortunately most commercial paddle makers have very little to no paddles sized for children. Most are appropriate for maybe young teens.

I make my Girls their own paddles. Each one is usually good for about two years. My nine year old is on her third. I make them a generic Greenland style. They don't use a lot of lumber and they are easy to carve. Each paddle takes at most a couple hours to carve and sand. For lumber I use solid western red cedar. They are oiled with some left over Watco and then waxed.

Sizing: I oversize them very slightly so by the end of the first season they are just about the right size. By the end of the second season they're usually just barely too small. Time for a new paddle. My 7 year old is barely 48" tall. Her paddle is 63" long. My 9 year old is 52" tall. Her new paddle is 67". Standing they can each barely reach the tip of their paddles with their fingers.