What was your thinking behind the gun, there are probably other solutions that are better.
Because a gun board is not really a good first real board, a 7ft gun is designed for big waves , it is likely to be too narrow and not have the basis shape to encourage the development of skills as an all round surfer.
If your son has been surfing a soft board then perhaps I'd consider something like a fun shape of about the same size, depending on his size, age and weight you might be able to go a little shorter but with more volume in the board.
However if you go too short and he has some serious growing to do, he may grow too fast and be beyond the board quickly.
My usual advice is to be a little longer at this stage and as he gets more skills he will also begin to know more of what he wants.
Hope this helps

I've taken up troll hunting just for fun, instead of a rifle I'll just use a pun! 冲浪爷爷