All reptiles carry salmonella. For turtles, it's a necessary gut bacteria and aides in digestion. Kids shouldn't be handling turtles. I won't even let my 11 year old nephew near them. They're a "look but don't touch" pet. If you wash your hands thouroughly before and after handling the turt or anything in the tank you'll be fine.
There is a law that turtles under 4 inches cannot be sold but there are loopholes. For example, they can be sold for educational purposes. Don't ask me what that means or how it works. It must be an American thing because I've never heard of it here.
If you plan on letting him keep the turt, it'll really be your pet. Read all the stickys on this site. It'll fill you in on the basics of what your turt needs.
Good luck.
The things that come to those who wait may be the things that were left by those who got there first - Steven Tyler