S.L.
My first venture to CA was with an 18 month old, 3 yr old and 5 yr old (although I have flown with them as babies. I recommend getting your son his own seat and taking your car seat. 5 hours in flight is a long time to deal trying to juggle your 9 month old in your lap - even if you take turns. Coming back will be even longer as you travel with the earth's rotation. My child was bigger and mobile, so this was more of an issue, but I believe they all napped longer and better in their car seats than they would have in my arms - and it gave us stretching room. If you didn't purchase an extra seat, you may be fortunate enough to request a row with an empty seat that you can put your child into if no one will be using it (provided you have your car seat). The extra space will also come in handy if you have to change your baby. I don't recall a changing station on the airplanes we were on.
Many people suggested to us to give our child Tylenol, or some infant decongestant (Dimetap or Triaminic) in order to preempt the altitude pressure change (plus it makes them a little sleepy), which we did, but our children are prone to congestion from allergies anyway. Since I don't know your health history, I would probably talk to your ped about it. Some children do just fine with the cabin pressure changes.
At 9 months, your child will probably be just happy enough getting your attention and looking around the plane. We took several books with lift-a-flaps or textures to feel. We didn't bring the noisy books out of respect for the other passengers who would have to hear them for the duration of the trip. We also took four or five favorite toys (packable) and a favorite blanket.
Try to be aware of in-flight times and scheduled meal/nap times. If your child will be ready to eat close to the time the plane takes off, you may want to bump up mealtime. I don't know if you've got your child on cups or baby food (another reason for a separate seat), but, again, pack your diaper bag accordingly. Oh!, and take along some diaper bags for those dirty diapers/wipes. etc.
To make luggage a little lighter, we did not pack food, diapers, and wipes for the whole trip. We took enough to get us through a 2 days (traveling day and the next), and bought enough in CA to get us through the trip and back home. If you are using formula, I recommend getting the premixed solution to have for the flight. It's not the cheapest, but it's easier than carrying a tub of formula and water bottles, then still having to mix it all up. Trust me, when you're on the plane, crammed in your seat with an infant on your lap, the less stuff you have to mess with, the better.
As for the time change, we didn't do too much to take our kids off of our schedule because, by the time they got adjusted, we'd be coming home to have to re-adjust them, and I wasn't keen on that. My sister-in-law who would travel from CA to MD, felt and did the same when she traveled to the opposite coast.
So, we pretty much stuck to our east-coast time table with the kids although we stretched it. We tried to keep our children up a little later then usual,to get them to sleep in an extra hour or two. It may take a little more effort with your 9 month old to keep him stimulated. Hopefully, since it won't be light at 4:30 am, you may have to nurse/feed him to assuage his appetite, but he will go back down for some more rest time. Our attitude was not to force a new time schedule but also to not enforce the old one. It was easy for my husband and I to adapt and curb our sleep/hunger for CA, but we made ourselves aware of the kids' internal east coast clocks. Sometimes that meant feeding the kids when they were hungry regardless of CA time (although not east coast). Fortunately, you still have an infant who's eating schedule and meals are separate from yours anyway, so this would not be a stretch for you, just something to work in your CA daily plan.
Hope this helps - enjoy your trip!