I would agree with previous posters in that the level of maturity should have a lot to do with the decision. Each person is different. At 17 I would not have been able to do a RTW trip or some such thing where everything was not already planned out. I did however at that age pack up and move to a country 3,000 miles away. My parents made a judgment call in letting me move and were OK with that.

Given specific times and destinations, with the proper precautions, I should think that this trip would be manageable. Who knows, ten years from now your son could be providing his experiences in this forum to someone else looking for advice.