I was a single mom with my son (with no dad involvement at all) until I got married when he was 8. He now considers my hubby his dad - and he was 12 when I had my daughter with my hubby.
Your son is old enough for you to talk to him about the situation - and maybe about his little brother with dad, too. It's best to do this when you're in the middle of something fun with him, or driving in the car, so he doesn't feel "lectured" or put on the spot.
Explain to him that babies are a lot of work, and it will take a lot of your time, and let him know how excited you are for him to be a big brother and help you out and show the baby how to do things. Most of all, make sure he knows this doesn't mean you will love him any less or that he's any less important to you.
Also let him know that you WILL still have time for him - and then you need to make that happen. Even though you're going to be exhausted, you need to set up time (when baby's sleeping or when someone can watch the baby) where it's just the two of you doing something fun and special, like playing a game or doing arts/crafts or reading together - something that doesn't happen when the baby's around.
Get him involved in picking things out for the baby's room, too. My son felt very important for having picked out which bouncy seat we would get, which blanket she might like, which bath toys she would like, etc. He even went through some of his stuffed animals and gave most of them to her. Since he's very good at art, I had him draw a special picture to hang in her room.
My son wasn't that interested in her until she could interact with him, and now he loves to play with her and help me with her (He's 15 and she's 3).
Your son is getting to the age where he'd rather play with his friends and will like that you aren't overly-focused on him.
Good luck!
D.