Well, it sounds like your husband doesn't want to spend the money on the airline ticket, right? As the date approaches, I'm buying a ticket now will be even more expensive than your own ticket...but you could inquire if there is an infant rate. The seats are so cramped on an airplane that it will quickly get uncomfortable for you to hold him in your lap and sleep even while he's sleeping.
For whatever reason, whenever we've flown outbound on an evening trip, it's been more difficult for us. I always hoped they'd quickly fall asleep since it's close to bedtime, but I think they get too excited or overwhelmed by being on an airplane that it never went as planned. We just flew to California (only 4 hours compared to 9) with our youngest child (almost 17mos), and it's just a very difficult age for kids to fly. They're too active and into exploring and don't really have the attention span for a movie. She didn't want to stay in her car seat, and while she was mesmerized by an episode of Sesame Street that we downloaded/purchased, she only watched because the headphones bothered her too much. That was the first time. On the return flight, she kept trying to grab the ipod and push the buttons.
Bring lots of snacks and even treats. Buy the single-serve milk boxes (Starbucks always has them); they're ultra-pasteurized and don't even require refrigeration until opened. Include his favorite books and quiet toys.
It will be a very, very busy flight for you and your husband to keep him happy and entertained..and that's even if he has his own seat. Your son will need constant attention and play.
As soon as you land, let him sleep the first day but curtail his sleep so that you can start getting him on local time.
Sorry -- I don't mean to sound pessimistic, but it's just best to be prepared and know what to expect. As I said, you can keep him pretty happy but it will be a lot of work, especially if he's an active child, because you have to keep him fairly contained on an airplane and there just isn't a lot of space for anyone.
Good luck!!!