I'm going through the process right now with my son. He just finished his sophomore year and made all league. He also plays on a summer travel team. We were down in Diamond Nation in June and again in late July. He had a good showing, hitting two home runs, stealing some bases, making some nice plays in the outfield. A lot of college scouts watch those games. Some scouts came right to the dugout between innings to talk to the coach about my son and a couple of other kids. Other coaches called the coach and left their numbers asking my son to call them. My son has called some schools back and been invited to their camps and to take visits. We had our first visit last week and he received his first offer from a Patriot League school. We're visiting an ACC school next week and his summer coach feels he may receive another offer.
I'm no expert and am just learning the process, but this is what i understand at this point. Your nephew can initiate contact with college coaches via email and telephone, send them video, express interest in the school, talk about his baseball accomplishments. They cannot call him or email him directly, other than a generic invitation to a camp. That all ends on September 1 of his junior year. At that time the college coaches can contact him directly.
So the important people at this point in your nephew's search are his HS coach and his summer coach. Colleges must go through them and ask them to have your nephew contact them. They also ask info about your nephew's talent. Your nephew can also make contact on his own but the college coaches prefer to talk to one of his coaches.
My suggestion would be for your nephew to send some video out to some schools he's interested in with some info about himself - height, weight, position, HS, summer team, coaches names and contact info. That way he can get on their radar early.