J.M.
It's tough being the oldest. And nine is getting to be a hard age - he's not a little boy, but he's not a teenager. He probably DOES wish his little sister wasn't around - because (in his mind) his life would be SO much better without the pesky little brat taking mom and dad's time and attention away from him, and getting into his stuff, and just being a big pain. (Been there. Done that. I had a little sister, too. And I just KNEW my life would've been "perfect" if only my parents hadn't had her!)
I don't think it's very useful to feel sad and uncomfortable about how he feels. Many older siblings resent the younger ones. But you have to remember that children are selfish little monsters who have absolutely no life experience. So, you and I both know that his life would NOT be perfect if he was an only child but he's just not mature enough to understand this . . . and he won't be for a number of years.
I've noticed that my oldest starts getting really grumpy toward his little brothers periodically. And when we fall into that trap, it's like he can't do anything right and he's getting into trouble ALL the time. What we do to break the "bad cycle" is to make certain to spent some extra quality time with him. I'll take him to the movies. My husband will take him fishing. We'll bake a batch of brownies, etc. I try to do something with him every day, even if it's just reading a ten minute story. Just some one-on-one time to make sure he sees how special we think he is seems to do wonders for him.