Hi L.!
Every baby is different, so it's hard to predict what's going to happen with yours on the plane. My 15 month old (at the time) didn't fly well last time we were on a plane, but I did learn a few things that I can pass along to you.
First of all, if it's possible to let your child have his own seat, then do it. I flew with my baby on my lap and she struggled the whole time. I think she would have been much more comfortable in a seat of her own, strapped in. If your baby loves to sit on your lap and is used to sleeping that way, then that's one thing, but my baby is very active and wanted to get up and move all the time. She understands that she can't do that in her car seat, but on my lap, she just got irritated that I couldn't always let her down.
I actually did give her benedryl for our return flight home. I called our pediatrician and asked her about it. She gave us her dosing information based on my baby's weight (she's only about 22 pounds, so she might be different than your baby) but she warned me that it doesn't work for all babies and it might have the opposite effect and make her hyper. It didn't do anything to my baby but frustrate her. She was obviously tired, but she was so stimulated by being at the airport and on a plane that it just made her even more cranky. You have enough time with a nine hour flight, I would suggest that you see how things go first and if he seems to need a little calming down, you could give it a shot.
9 hours is a long flight for even adults. I would suggest when you can, you get up and walk up and down the aisles with him so he can stretch his legs and you can too. Make sure and take lots of snacks, drinks, diapers and wipes. The plane is a hard place to change a baby, but with that kind of a flight, you'll need to. You might practice changing him while he's standing up so you don't have to find some place to lay him down on the plane. Once you get good at that, you can walk him back to the lavatory and do it fairly quickly.
If all else fails, just remember that you're going to Hawaii and this will be an amazing adventure for the two of you. Even if he does scream the entire 9 hours (which I'm sure he won't) you probably won't see any of those people again. Everyone was very supportive on my flight. Try to maintain your stress level so he doesn't stress because you are.
Have a wonderful and safe trip!
Hilary