I just recently found my birthmother. Several years ago I made a post at adopteeconnect.com. Her sister put in my birth date and the city I was born in and read what I had posted. Details matched so she emailed me. It took me 6 years to find her. I suggest your friend put his information in as many databases as he can. There are some that you have to pay for but there are so many that are free. Also, I know my father's name and the basic area of his location and cannot find him at all. So I would suggest making sure that he can be easily found. For example, keep his home phone number listed under his name, in case his son has his information. Also, he should call the agency that was used and talk with them. It's possible that they could keep a letter from him and give it to his son if the son contacts the agency. While the adoptive parents seem to be the most direct route, I would be hesitant to rely on them. My parents are great and have been nothing but supportive about me reconnecting with my birth family. However, not all adoptive parents are supportive or they may feel he isn't ready, etc. 18 is very young (says the 23 year old lol) and while I was VERY intent on finding my bparents at that age I am now glad I didn't. Even though I wanted to, I wasn't ready and I wouldn't have been able to handle that type of relationship at that time.
Also, I wish him luck. From what my bmom has told me, she met a lot of dead ends. The agency that I was adopted through sent her in circles and was very dishonest. They also gave her this big lecture about how I most likely would not look for her until I was in my 30's and had a child of my own. He has options and I hope he finds his son!