Snow tube, plastic sled, or old school wooden sled?
I do think its fine. As long as the hill is relatively small. You don't want him going down a super luge or anything cuz he is still small and his lack of weight can launch him too far into the air if anything freak accident happened. However a small hill isn't a big deal. They are so resiliant at that age that even if he falls off the sled he'll be fine and probably would enjoy that more than the sledding itself! Boys you know?
I had both plastic sled and old wooden sled... loved the plastic sled though
The best sled I ever had when I was little was one that my step dad made.. It consisted of two old snow skis, and a small wooden seat nailed to them.. and a rope tied on the front of each ski, creating a loop. It was much better than any sled I have ever had.
Well out of the ones you listed, I would say either the tube or the plastic sled. Tubes are cheap, and when you land if you go off of a jump or something, you have more cushion. However they can be harder to stay on. Plastic sleds are cheap, and durable, so if you run into a tree or something it wont pop. Plus they are easier to control and steer. They are just kind of rough to land on. I bruise my tail bone every time I use one, but its fun.
Tubes, this is the most ingenious invention since sliced bread!