Most of you are wrong here and advising on what you want NOT would be best for the boy. Semi- autos are wrong, wrong, wrong.
A person needs to learn on a single shot or close to it. That way they learn to make the first shot count. A lever action would be a good second.
My first gun was a single shot .410 and later a .22 marlin 39A which I still have. Used Marlin 60’s are worth about $60 around here.
My first choice would be a lever action Henry which will still be a good rifle 50 years from now.