I suggest a Benjamin. I have a well-used 347 that I would never part with. The only drawback for a young shooter is the last two or three pumps, and Dad can provide that. The general advantages: only wants to be shot and be given a little oil every now and then. The advantages for squirrel hunting: enough power, light (he'll be carrying more than pumping and shooting), and compact (boy-sized).
Of course, an 8-year-old boy probably has an idea of what he wants. Ask him. Or, better yet, let him look at, and hold, various guns and then ask him.