Will he be racing? How big is your yard, or area where he will be riding in?
My first "real" motorcycle was an XR 100 and I loved it. However, it only took a year for me to want something faster with more suspension.
If he's going to race, then I'd say go with the SX65. An SX65 is going to do wheelies a LOT better than an XR50. It will also be a LOT faster. He (or you) will have to mix gas for this bike too. As a parent, you'll have to learn to step back and let him crash once or twice. If he's going to ride a motorcycle, a crash...lots of tip overs, and crashes, are going to happen.
Right now, a lot of the 11 year old riders in my area are on 2-stroke 85s. I think this would be a little too much, in light of the fact that they are pretty hard to learn on.
If he's riding around the yard, I would probably suggest a CRF150F. You can find them pretty cheap for now, and might last a little longer than an XR100 powerwise. It's not a racing fourstroke, and you won't have to mix gas. He won't have to worry about dropping the clutch quite as much, and it won't take off on him as suddenly as the two-stroke 65 would.