My maxi is cut to 16.5 inches, and I love it. You don't gain a heck of a lot in weight loss cutting the barrel, but it does make it Handier. If you do cut the barrel, I'd ream it to max at the same time. He can shoot the maxies when he grows into it.
I'm a little confused. You say the length of pull is OK for him, so he's got to be a pretty lanky seven year old. On the other hand, you say the weight is too much. I would think you need to double check the length of pull too.
Pete