You seriously need to back off on your son. He needs time with just his parents, not with the baby. That means that some 'family' times will need to be centered around him, and some around the baby. I mean, how can the baby participate in any sort of 'family time' your son would enjoy?
I remember being eight when my mom had my younger brother. I had a sister who was six at the time. My mom had the same hopes that you did and I was made to feel bad for not helping with certain things, which was dumb, because my parents chose to have the baby, I didn't.
If you are a good parent, having another child was not a 'family decision'. Even if parents 'think' their kids want a sibling, children have NO idea what comes with infants and toddlers and it's not fair to expect them to do what we feel is right.
Think about it... two parents are now far less available. They expect him to do things that seven year old boys aren't usually excited about in general. (I know very, very few boys at this age who would be happy to pick out baby clothes to play with or select baby friendly activities.) He needs to be able to have fun with kids his own age. He needs you to find a mother's helper to hold baby for a little while so he has time with you that is centered around him and not his new sibling.
Six months is a very short time to successfully make a huge adjustment. We can talk up the new baby, but that may also backfire-- some kids just find it to be too much 'in your face' to be helpful. Older sibs want to see you as excited about them as much as you are about baby's milestones.
So, my advice is this--- let your son have his authentic feelings for a while. If he's ignoring the baby, it's okay. It's not his baby to worry about. At least he's not hurting the baby-- and that's a point I would say "get professional help". But it's not there. Stop trying to make him be interested in something he isn't. Consider-- at age seven or eight, I really couldn't have cared less about a baby or doing 'baby centered' things. I did want my mom back, because she was always doing housework or holding my brother or something like that. I wouldn't have been happy to be forced to play with any sort of younger child. Just not my interest at that time. That's being realistic.
Let your son know he's okay if he's not interested in the baby. There's no law that says he has to be. Don't make him feel bad for it. Remember, too, that you are projecting a LOT onto your baby as well. Babies can ogle someone adoringly but it doesn't mean they will be devastated if that person ignores them. Your baby is not sitting there thinking 'brother didn't get my clothes, brother doesn't like me'. Baby is still needing YOU, primarily, and really, nothing more. Don't put those needs and expectations on your oldest.