This decision depends A LOT on how your son travels. I've been travelling alone with my son since he was 6 weeks old, and lucky me, he's a GREAT flyer!! We've never bought a ticket for him, and we're totally dreading having to come October because like you said, flights are ridiculous right now! Here are some other tips that have worked for us.
1. We almost ALWAYS fly Southwest because there are usually empty seats and it's festival seating, which means you don't have an assigned seat. I usually ask the flight attendant as we board if the flight is full, and if it's close, he/she will tell you that they'll let you know if they need the seat next to you.
2. I usually sit towards the back of the plane (where no one else really wants to sit) by the window with him or my jacket, etc in the middle seat, and if someone wants to sit on the aisle, that's fine. Only once in 19 months have we been on a full flight. Other airlines are another story....
3. I try to schedule our flight early in the morning or around naptime, so he would nurse during take-off and sleep for the flight. I put him in the sling and have a great flight! Festival seating also works for nursing because you can sit against the window on the side of the plane that corresponds to which side is next.
You won't be able to put the tray table down regardless of his size, but being lucky enough to have an empty seat next to you allows you and your child to have their own tray tables. 75% of flight attendants LOVE kids and will help in any way they can.
I wouldn't recommend putting 3 kids in 2 seats because of the armrest base, but I also don't like the idea of strapping my kid in his carseat. He loves to climb around and look out the window and stand in my lap so he can see the other passengers. We usually get lucky and have a couple around us who like to play peek-a-boo or just talk silly to him, and he laughs!
I'm also one to try almost anything once. With your child being 20 months old, even if the flight is miserable, it'll probably be the only one because he/she is so close to turning 2 anyway. I wouldn't buy a ticket unless you get a good price on your flight. Good luck!