With car insurance being what it is for young drivers now, a 125 at 17 is not a bad option, something like a cbf125 is good, easy to handle for an inexperienced rider and will return 100mpg. On a side not it may also be worth doing a car test even if he won't be driving, a friend of mine went 3 years between passing and actually getting on the road, by which time he was older and had held a licence for three years, his first years insurance was a fraction compared to the rest of us despite having no more experience.