A couple of things. I don't fully agree w/ people saying he doesn't need a father figure, although this term can be interpreted differently. He does need healthy and strong male influences, esp if he's an only child. So it's not about finding "a" father figure. It's more about role models.
Sports and scouting are both ideal for this. This includes sporting offshoots, like Indian Guides, etc. Also, don't forget about extended family. My sister ended up divorced, and she made sure her son had heavy contact with our brother-in-law, who is a terrific dad.
Finally, and maybe most importantly, you say his dad isn't a good role model, and this may very well be true. However, odds are high his dad will want and get visitation, so you need to walk a fine line here. It will be much better for your son if you and his dad remain amicable and try to get on the same page as much as possible. This means you pick your battles. If he's a weekend dad into fast food, this isn't a battle. Save that for more serious issues, and when you talk to him, try to avoid pointing fingers. Instead, keep it neutral and try to talk about how certain behaviors impact his son.