You also have to remember that he is a boy and boys don't care about babies. I'm pregnant with my fourth and after hearing the heartbeat we told our 8 year old son and he said " ok I don't really care about babies" I would just prepare him now, letting him know that you love him very much but will need his help when baby gets there. Maybe have him start now by giving him tasks to get things ready for baby. Just keep reminding him that baby won't change how you or his dad feel about him. Don't stress! It is a challenge but you will all get through it.
My son really hasn't had much response to the pregnancy. When we told the kids my daughter was so excited and since then she's been asking all types of questions and just wants to be involved with this pregnancy. My son on the other hand just seems completely uninterested. I'm not sure how to really take that response
I have two older boys, ages 8 and 3. When I was pregnant with my 3 year old, my then 5 year old loved to be a part of the weekly pictures I'd take of my belly shots, and enjoyed feeling the baby move and talking to my belly, and one time he and my husband took washable crayola markers and drew a globe on my belly and watched as the baby inside created "earthquakes." Once the baby was born, he wanted to help out in any way he could, from picking out clothes to brushing his baby brother's hair after a bath.
This pregnancy is pretty much going the same with the reaction of my now 3 year old. He is always talking about the baby in my belly, and how he thinks he has a baby in his belly too (and he loves that my husband and I play along with that idea). He is actually more "annoying big brother" than my older child was to him in utero, lol--he likes to poke at my belly or gently push it while I'm lying down so he can get the baby moving around and kicking his hand. That cracks him up. My older son, in response to this pregnancy, is excited again as well...but it's more reserved this time, kind of like a been-there-done-that kind of reaction.
I consider both my kids "clingy" in their own ways with my husband and I. My oldest has always been a mama's boy and my youngest is a daddy's boy and VERY attached with his dad. However, we try to keep things as balanced as possible with individual time we give them for their own benefit and family time together to encourage our interaction and bonding together without jealousy issues becoming a problem. Not that jealousy is never a problem--kids will be kids and they will get jealous of each other and have sibling rivalry from time to time, or feel one is more favored by a parent or both parents. That is inevitable.
But the best thing you can do is just be sure to encourage family participation and interaction in ways that interest the kids, while still being sure that you set aside time for them to have you all to themselves for a bit now and then. :-)