I live in the Santa Cruz area, so skateboarding is big here! I would avoid going the ToysRus, Wallmart, Target route. The wheels and bearings on those boards are very cheaply made and extremely frustrating for riders. You'd think the 'slowness' of these wheels would make them safer by limiting the speed, but actually they cause more spills with their uneven motion and inability to deal with small obstacles, like twigs and such on the pavement. As far as size, brand, etc. With the exception of specialized boards for racing and such, there really is only one standard size. An 8 year old would be fine with that and able to develop his skills and tricks. Board brand has more to do with design/color/logos etc. A matter of taste. Smooth running wheels and bearings are key though, so a specialized shop is best for this sort of thing.
I don't know much about the Sacramento area, but I'm sure you would have a decent shop. In this area I would highly recommend Santa Cruz Skate on 41st Ave.,They really know their stuff and gave us excellent advice with our beginner. It was clear that everyone there loved this sport and were not just out to sell you something. They really are very focused on the younger kids there and helping them get off to a safe start. Injuries are bad for business! Maybe a birthday beach trip?
Eight years old is plenty old enough for a board as long as he understands that if he is caught without his helmet he loses his wheel privileges for a month. (wheels meaning bike and scooter too! That is still the rule at our house and my kids are 13 and 15). Not even down the driveway without a helmet. :)
Good luck in your continuing adventure!