Six-year-olds don't need their own portable systems, but if you must, get him a Nintendo DS. Dint get one of those 3DS's because the 3D can damage the eyes of kids his age, and they're extremely expensive. I think that the original DS's are below 100 dollars now.
The new Nintendo 2ds made for kids of the age 7 and under. They just made it because the 3d visuals on the 3ds can harm child's eyes. The 2ds is ideal because they can also play the games the 3ds plays, just not in 3d.