It is good for all children to have both parents in their life where possible,his Dad made an effort the see his son once, perhaps for your boys sake you could find him and see how thing, It is up to you , in the interest of the child I think it would be great for him to have his Dad around and get to play sports and Games and go visiting. If you know where he is you could pay a visit and see what he is like now before you tell your son, Good Luck, it is good you are considering it and have seen your Boy would like a Dad around.
I don't know what to advise about his father. Would he be a bad influence. Is he the kind of guy that you just don't want hanging around? Is it at all possible that he could have a good relationship with his son regardless of how you both feel about eachother? I think either way you are going to have to explain things to your son in simple terms. Your son may resent you for not allowing him to see his dad. You will have to explain that if he does see him that his dad may be totally unreliable and may continually break plans etc.
Aside from his dad, have you tried getting him involved in things like scouts where it is all boys and men together? Other activities that are very good for a broad age range of males are things like rock climbing. Are there any other family members eg. brothers in law, grandads etc that he can spend some time with?
if it were me i would not bother the father cause do you really want him to bounce in an out of your sons life that would be alot worse they make a book although i cannot remember the name of it about families and how they are different some have mom and dads some have grandparent some have foster parents etc.... right now is a great opportunity to teach him to cope just do not lie to him be completely truthful with him he will understand