Heck at five. I'd just give him one disc and see how it goes. I gave my daughter a starter set when she was around that age. It turned out to be to much. She had more fun just throwing one disc without the worries of sellecting a driver or a mid or a... So she just ended up throwing a Teebird for a few months and had more fun doing that. Kids tend to throw for a few holes and then drift off when they spot a bug, stick or a cool plant lol! At least that's been my experience with kids.
My son started playing at age five by trowing what was in my bag, but I think kids do the best with under-stable discs. They have not developed the arm speed, or strength and a disc that can turn, then come back will create the most distance. I agree a "Roadrunner", or maybe a light Star TL. I don't think it could hurt to start with lighter disc, until they develop, so let us know how it works out. Love to see our children playing. I know my son is 19 now and can usually beat me, mainly because he started so early!
If your looking for ultra light discs check out discraft's misprint page. If I'm not mistaken for 10 its like 35 or so bucks plus shipping. You don't get to pick your discs but for cheep he can pick thru and find a few that will work for him!