My advice is to start from scratch with a rifle that fits your son. My own son got his first rifle last year when he turned 9. We had a gun show here in Tahlequah on his birthday, and rather than buying him a "surprise," I surprised him by letting him go to the show and have a voice in picking out his first rifle.
At the show, I tried to talk him into a Crickett .22LR. He wasn't terribly enthused with it at the time, and claimed that the rear sight was useless because he couldn't see anything through it. He also didn't appreciate the fact that they are a right-handed rifle and he is left handed.
He tried out a Rossi youth model "Matched Pair" single shot with interchangeable .22LR / 20 gauge barrels. He immediately liked the Hi-Viz open sights better than the peep on the Crickett. The rifle seemed to fit him okay at the time, so I let him have his way.
At the range, it didn't take long for me to realize that the Rossi really didn't fit him all that well. It is also a fairly heavy gun, and it was difficult for my son to control. I enjoyed shooting it, because it has a surprisingly decent trigger and is also surprisingly accurate. My son found it difficult to hold steady, and almost impossible for him to reload and cock by himself. On subsequent range trips, he began shooting my 10/22 more and more. He could control it a little better because it is lighter than the Rossi. He hit a little better with it, but not by much. It was still too much gun for him in terms of size and weight and bulk.
Shooting with his rifle or mine was something that he was willing to do, but something he could "take or leave." I think he felt obligated to try to like it, since every male member of his family on both the Caucasian and Cherokee sides are avid outdoorspersons, and some of the ladies, too -including his mother. But I wouldn't say that he had "independent interest" in shooting.
This year, when his birthday rolled around, I took him to Wal Mart to have him try on a Crickett for size again. This was, in part, his idea, as he had previously mentioned that he might have made a mistake in choosing the Rossi over the Crickett last year. He also said that he now understands what the sight picture is supposed to look like through the peep, thanks to a WWII shoot em' up computer game he likes to play, and that he didn't really understand what he was supposed to do with a peep sight before, in spite of my best attempt to explain it to him. When he tried on the Crickett this time, as soon as he got it in his hands and put it to his shoulder, it was like a light bulb went off in his brain, and the expression on his face said, "Ah! So THIS is what a gun that fits is SUPPOSED to feel like." He was ready to buy, on the spot. They had a red, white and blue laminate stock model with stainless barrel and black blued barrel on black synthetic. He liked the black on black. I told him that I would buy it, but I wanted him to see a stainless and brown laminate version at the local gun shop first, and if he still liked the black on black best, we'd buy that one.
At the shop, he seemed to really like the look of the brown laminate / stainless model, and was amped to buy until he saw the price tag and noted that the version he held in his hands was almost a hundred dollars more than the black on black one at Wal Mart. After some discussion about how I earn money and therfore feel some sense of entitlement to determine where and how I will spend it, and that the difference was worth it to me if he liked the more expenisve one better, he decided to go for it.
Later that day, at the range, I found that it was indeed money well spent. I fired the first shot to check for sighting and found it was sighted in by the factory to be dead on with the ammo we had (Mini Mags) at 35 yards. I showed him how to operate the rifle and his first shot was an X ring hit. He then proceeded to go through an entire brick of ammo. We ran out of ammo before he was ready to go home.
The next day, the first words he spoke to me in the morning were, "Gee, dad, it's a really nice day. I think it would be a great day to do some shooting. Can we go out to the range today?"
"You betcha. If I can some bullets for sale in this town......."
I bought all that Wal Mart had -about 600 rounds, total. He shot another five hundred of them. He was no longer using his monopod rest, but "position shooting" from standing, kneeling, and prone. By the end of the session, he was working on punching the center out of a 50 M smallbore target at 50 yards. His sustained rate of safe, accurate fire with the little single-shot Crickett has to be seen to be believed.
Once again, we ran out of ammo well before he was ready to quit.
Since then, we've been going to the range after school, on days when we don't have some other conflicting prior commitments. And we still run out of ammo before he is ready to quit. And he still asks to go to the range every single day. He has also decided that we should move out of town into the country, on a piece of land big enough so that he can have a safe place to shoot his Crickett every day. He is also 4'3" tall and weighs 62 pounds.
The long-winded point of all of this is that there is a difference between a gun that fits which the shooter can control, and one that doesn't fit too well. The former is a source of frustration. The latter is a pathway to a lifetime of wholesome fun.
Even if you have to buy another gun to get one that fits your kid and your kid can control, the end result is worth the price, IMHO.
-JP