if you want inexpesive, the new mossberg or marlin mod 60 is fine, the mossy is actually a bit of a copy of the marlin. mossy also makes that same version in a nickel washed steel. If you want even cheaper, go to a pawn shop and find the marlin mod 60, and pay somewhere between 50 to 75 bucks, don't be afraid to haggle. As for scopes, don't get used, go to a big box store, like Walmart, and get a scope. it will be 3x9 power, make sure it is in a package that allready has the scope rings/bases. it will be 30 to 40 dollars. But before you use the scope, make sure you teach your son how to shoot and adjust, with the open sites. if you don't know how to , any range officer at any range will be glad to help you, or heck, we will tell you how to do it here! Then finally , get a cleaning kit, and a bulk pack of ammo, cci blazers, or federal, it will be about the size of a brick, that is why it is called a 'brick' of ammo! if not a brick, then get one box of every type 22lr ammo they have, and once you start using the scope, find out which one your rifle likes best. If you dont have a range bag to put this stuff in, plus some rags , tools, targets, shooting glasses , ear muffs,etc. , get the cheapest nylon or canvas one you can find, keep it under 20 bucks.
Alltogether, you can get everything you need, right from the start, for about 150 to 170 bucks. Teh most important part will be to go to a gunshop or a pawnshop, and get your rifle as cheap as you can! Like I said, 75 bucks, with tax included , should be your ceiling.