1) yes you have to have a street legal bike and rider to ride on the Forest Service roads. There are ORV areas/parks where you don't have to have a license. Whissenhunt near Dahlonega, Houston Valley near Lafayette, and there's another near Clayton that I can't remember the name. The biggest issue with most of these places are they are heavily used by ATV and SXS's and most of the trails are two way, so be prepared. There are two private ORV areas in Georgia, Durhamtown Plantation that's east of Atlanta and Highland Park west of Atlanta near Cedartown. DP is more oriented towards beginners and ATV/SXS riders. They have a bunch of rental bikes from small to large. Might be a good place to go try a bike and see what fits and he likes. Highland park is more trail oriented and has a variety of trails. The trails are marked 2-5 diamond for difficulty. They also rent bikes (normally) and gear. Their bikes tend to be nicer (KTM's) as they are also a KTM dealer and you can "try before you buy".
2-3) With the Yamaha, make sure you are looking at the right version. The WR250R (notice the "R") is the dual sport bike. Fantastic bike that will certainly grow with him as he gets more into riding. I have quite a few friends with the "R" and they are solid transportation and recreation. The WR250F is the off road race bike. Very different bike and very high strung and maintenance intensive.
don't let the seat height scare you- most of the bikes have a lot of "sag" so when you sit on it, the seat drops an inch or two (or more). My dual sport has a 38" seat height. I'm 6'1" but it's fine. technically if the suspension has a linkage you could lower it with a new "dog bone" and then raise it as he gets more comfortable or grows. Most of the brands out have a good entry level bike these days. CRF from Honda, KLX from Kawasaki, DR or DRZ from Suzuki etc. Each has little areas that they might be better (suspension, fuel injection, weight, fit and finish, etc) but all of them are pretty capable and should be a TON of fun. Typically the "play bikes" are heavier, cheaper suspension, more docile motor, and great for beginners to learn the ropes.
Buy a used bike and ride it a while and see what he likes. If you need to you can sell it for close to or more than what you paid for it and get whatever fits his needs better.
Licensing- I would HIGHLY recommend having him take the GA MSF class. It's a full weekend but they provide the bikes (street) and if you pass the class you get your license. BY FAR the easiest way to get your license. Plus you may get a discount on your insurance if he buys a bike.
I have had a few friend recently that wanted to get into dual sport riding to go play on the weekends with me. So i wrote up a little blog post about getting into riding and then more specifics about dual sport. It's oriented more towards KTM bikes as that's what I know best and IMHO they are hard to beat if you plan to ride a lot but the info is pretty solid and there are some details about gear, other brands, etc. that might help fill in some gaps. Take a read.
https://www.planetadams.com/blog/
sorry for the long winded reply, motorcycles are another passion of mine and I love to get friends into riding, gives me more folks to ride with..
