Many people might say why would you become "best friends" with someone so much younger than you, and why wouldn't your "best friend" be someone closer to your own age. Aside from genuinely liking each other as people, some might wonder about your intentions, they might think that your interest in the boy is more than friends, that maybe you can't make friends your age or that you simply want someone to control or impress.
Your question was, is it an issue. It's only an issue if any of the above are true. If his parents don't mind you being around him, and you have a good relationship with them, then that's all that really matters. If you look inside yourself and know that none of the reasons above are true about you, and you genuinely feel like a big brother toward him, then I really wouldn't worry about it.
Do keep in mind however that as an adult, you can do a lot more than he can. You are also someone he probably looks up to, so be sure that you are not affecting him negatively, or doing things you know his parents would disapprove of. In fact, whatever his parents say, you have an obligation to back them up, even if you don't necessarily agree with them. His parents are entrusting that you are not going to lead their son off of whatever path they have set for him. Your friends are going to think that, for some of the reasons I mentioned above. Perhaps you should let them know its a "big brother" type of deal and they may leave it alone.
Its a very special thing to have someone of that age accept you and be their friend, so treat it that way, and be a good role model for him. I welcome your thoughts, let me know if you want to talk more.