I would use the conversation as an opportunity to validate his feelings, talk about practical solutions that would help him to feel closer to his biological father, and set very clear expectations on what dating would mean for him. Let him talk about his feelings of loss and of being jealous that his friends have dads who are here who can do things with him. I was a single mom for 5 years and I know that hearing these things is painful but as strong mommies we have to let our kids know that we can handle their feelings of loss - it sounds like you two have good communication, so just make sure you continue that. Sounds like you may have to add in something along the lines of "it sounds like it would be great for me to meet someone new and get married and you could have a step-dad right here, but real life is a lot more complicated than that. It's not like how you see things on TV and in the movies, where parents marry someone new and everyone lives happily ever after. It would be nice to meet someone special who I would want to be a husband to me and a step-father to you, but that might not happen at all or would be years and years from now, and no one will ever replace your dad."
Then instead of just focusing on the negative, see if there is anything practical that would help him feel better - more visits with his father? More frequent phone calls or Skype sessions? Is there a friend who has an involved dad who can include your son in some "father-son" sorts of activities (scouts, sports), or do you have a relative who can take on that role more actively? Is there a Big Brother/Big Sister chapter in your area that you could have him participate in?
Regarding dating. I think it's important to let him know that when you start dating, you'll spend a lot of time getting to know someone as a friend first to make sure that he's a good enough man to take seriously and only after that would he meet anyone you date. That dating is between the adults only and is something that you would do because it's fun for you if the right person comes along. Let him know that most dating relationships are not serious and end, and that you won't have him getting attached to someone who isn't going to be in your life for a long time. I think it's important to be crystal clear that not everyone you date will be husband and dad material, that when you are ready to date you will meet some great people and some you end up not liking and that you'll always put your son first.
You'll want to keep it simple and age-appropriate, but they do understand more than we think. Good for you for NOT dating and not exposing your son to other men. When you are ready to date, be sure to keep your dating life private so that your son doesn't get attached to everyone you meet. From what he said to you, it sounds like he would be very prone to getting attached and you of course want to spare him from having his hopes raised and dashed. Be sure to keep expectations really, really low.