I think what your son is sensing is your stress and anxiety over his feelings. If you are "walking on eggshells" around him b/c you are trying to get him to be happy about the new baby, he can sense that anxiety. He is entirely too young to understand what's going on - but he really doesn't even need to at this point. 8 months is a LONG time for him to wait to see this "baby" you're talking to him about. If you keep talking to him about it now, it'll just make the waiting period even longer and more stressful to him.
While you may think that you are trying to help him be happy about the baby, all you may be doing is harboring hate for the baby. If you start now, and talk w/ him about the baby "this" and the baby "that" and "this is for the baby" for 8 months, he is going to be very resentful of the baby.
Just go along your normal life, keep his schedule the same, and when you start really, really showing, briefly talk w/ him about why. Keep every conversation brief. Don't tell him more than he needs to know, b/c he doesn't need to absorb it all. If he asks a question, give him the short answer, only explaining more if he continues to ask.
Don't try to change his feelings also. For example, if 7 months from now he says, "I dont' want you to have a baby, I hate the baby" -- DON'T get angry at him, that will only make him hate the baby more. Just tell him things like "I'm sorry you feel that way" or whatever, don't discount his feelings, understand he is just a toddler and doesn't understand what he is saying.
As you get much further along, include him in buying baby things, but don't make that the focus of each of your outings. And be sure to get him a few little new things here and there also, wether it's a toy or playtime at McDonalds.
I'm sure it's stressful, but don't stress around your son, it won't do any good anyhow. He's got 8 months to mature himself and learn to talk and to understand the situation. He probably does not know what is going on right now at all other than Mommy and Daddy are upset so he is too. He is at an age where he lacks verbal communication, so he is really picking up on ya'lls anxiety over his feelings. just let it go, and it will be fine :)
Good Luck, God Bless and Congrats!