This is actually happening more and more these days. The scouting bureau is concentrating more on underclassmen. MLB clubs are hiring scouts to follow underclassmen. And we all know how college recruiting is working these days.
So I don't see anything wrong with having an advisor, provided that advisor proves to show an interest in the player. Most important is trust, remember there are a lot out there that will disappear as easy as they showed up in the first place.
The best advisors/agents truly put the players interests ahead of their own. In the end both benefit by doing this. I would always consider those that have been the most successful. You want someone that can afford to be honest rather than the guy that needs a quick pay day. Plus they have more contacts and can find out more.
So does anyone actually need one two years ahead of the draft? Most probably don't, but I don't see it as a real problem. So much depends on who that advisor is, how helpful he can be, and how much you trust his advice is in your son's best interests. At the same time, always be in charge of the situation, because nobody is going to care as much as you do.