Why wouldn't he be able to handle your 12 ga? I was getting my son his gun when he was 9 years old and around 70 lbs. I looked at the junior shotguns, but decided on a regular pump 12 with two barrels (normal and turkey). I'll never forget taking him directly to shoot five stand. The guy working the controls made a comment it was as big as he was. LOL. I said, don't worry, he'll figure it out. Guess what, he did. I told the guy not to push the other clay until my son completed the pump. Half way through his first round of 5 stand, he said just go back to normal launches. He hasn't looked back since.
I looked at it this way. A junior gun would fit better now but for how long? It costs the same amount and you know you will want to move him up sooner than you think. The 20's kick a lot of times more than the 12's, so they were out. I bet your son would do just fine and the 12ga will give you more versitility to do other kinds of hunting.
FYI, my son just said get him a 10ga.