J., I've been there and my mom's been in your shoes. I can tell you that my mom's biggest fear did not come true (me leaving her/me viewing him as a better parent/me liking him more). I can also tell you that the best thing you can do for your son is to support him. For years, my mom would cry when I asked her questions, so I gathered the very little I could and 8 years later I finally located him. From my point of view (prior to meeting him), he was going to be the perfect father, handsome, strong, great father to those he knew, pretty much the father I always wanted - even tho my mom tried warning me not to expect much). For me, if I didn't think he was going to be great, I would never have tried to find him.
The reason I and I would assume most kids want to meet their biological dad was to a) see if there are any resemblances b) find out why he left (was it because I was a girl and he only wanted a boy), c) did he have any other kids and if so, did I look anything like them d) medical history - what if my dad/his family had a history of cancer/diabetes e) to find out how he, as a dad could possibly live his life knowing he had a daughter out there and not once want to check up on me to see if I was okay and e) get an apology.
Once I actually found him, my mom never once said anything negative about him. She just let me do my thing and in my eyes supported me (I guess I showed her how determined I was and felt there was nothing she could say to make me stop).
But, when I was 21, I finally found him and it was a total emotional roller coaster (for me). I couldn't bring myself to call him dad, and I'm guessing if your son finds him, he too will have an issue with the whole "dad" name.
I got all my answers except I never got an apology. I have however met 6 of my brothers and sisters, the other 14 kids, like me, he's never kept in touch with (nor has ever paid child support). Anyway, it took more than 1/2 my life to find him and less than a year to choose on my own to disown him and finally say good bye. I also lost contact with all but one brother and sister-in-law.
By speaking to the 6 that I have met, I can tell you without a doubt that all those years that I thought my life would have been better only if he was around, was the total opposite. It seems that all the kids he has kept in contact with him had major problems because he was part of their life. Several of them tried committing suicide, they grew up with him drinking and into drugs most of their childhood lives and besides that, weekly unbroken promises. None of the kids ever went to college and most had kids of their own by the time they were 18. Ironically, they all seemed to idolize him and they were always trying to get me to believe that he wasn't as bad as I viewed him to be. But, after more than 12 years of no contact with him (that I don't regret), I have come to conclude that the reason he kept asking me why I was calling him (which was my attempt to try to get to know and establish a relationship with him) and what did I need, was because the only time any of the 6 contacted him it was because they needed money. I on the other hand was taught by my mom never depend on anyone for money, so that was the last thing I would want from him. But, it is what it is, he obviously never got a chance to know me since he assumed all I wanted was money and not a chance to get to know him as a person.
I have learned that the root of his problems came from his mom who felt her son could do no wrong and that "if I really wanted to meet him sooner, I would have searched harder" - yeah, her exact words.
Your son is determined, so the only thing you can do is support him and let him know that no matter what, you will be there for him. Now, if for some reason, your ex turned his life around and has a really good reason for not being around (which I doubt), you have to let your son and his father try to establish a relationship without trying to make it bad. You have to promise yourself that unless he is a bad influence on your son, you can't talk bad about him. Your son will be extra sensitive to anything you have to say in regards to him.
Keep in mind, you are the one who was with him from day one. It's perfectly normal for kids (especially at that age) to feel angry and insecure because the bottom line is that they feel they weren't good enough for their dad and if their dad can't love them, how could anyone else? I know, from a mom's point a view - it's possible, but personal experience, it's a question he deals with daily.
Your son needs someone who has been walked in his shoes. I know there was not support group when I was doing my search and I doubt there is one now, but if you or he needs to reach out, please feel free to email me and I'll respond. I know not all reunions don't end the same way, but it would have helped me knowing someone who felt the way I did when I was going thru what your son is now starting to venture into. He is truly getting on a roller coaster and he has no idea how many loops and dips it will have, nor will he know how long it will last, but one thing for sure is once he gets on, there is usually no turning back. He already bought his ticket and the only thing you can do is make sure he is strapped in tight. Cry when he cries, laugh when he laughs, just be there for him. It will be hard on you too, but know it's his choice. Now, you can choose to tell him when he turns 16 you will provide him with more details, but if you do that, he may be angry at you for delaying his search.
Good luck and let me know if I can help.
P.S. For me, it didn't turn out to be good, but on the flip side, I set out to find him, and to get to know him and I did so it was a huge accomplishment regardless that he didn't turn out to be the father I always dreampt about.