Hey Guys, That is a great idea to get him started as early as he shows interest. When I started it was with a stick welder, but I truly don't see any reason you couldn't start him with MIG.
I agree with the others. Start him just running beads. Once he can get a consistent bead, draw a line with soapstone and let him follow that. Once that is good let him spell his name with weld bead, or draw the robot he was wanting to make. That will give him a trophy to show off to all his friends and family.
Same thing goes with torch or plasma work when the time comes. Making strait cuts, following lines, then graduate up to his name.
As far a projects go,
bugs, cars, trucks, any thing that will keep his interest.
Great job for getting him started early.