You poor thing - what a handful you have! Pregnant and a toddler! Whew!
I have heard a few good ideas.
Since the air pressure changes can cause ear pain, give him some children's non-aspirin pain reliever about 20-30 minutes before take-off. Something to eat or drink while the plane taxis down the runway before takeoff and a few minutes before the start of your descent will help too.
DON'T worry about what other people on the flight are thinking, you will not be likely to ever see them again. Instead focus on your little one. If he is fussy, focus on making him more comfortable for him, not because he is annoying someone else.
Carry a comfort item, such as a favorite blanket or stuffed animal.
Another must is if your child takes a pacifier, bring extra.
The idea of news toys is great. Present them one at a time during the whole flight.
A good idea for a snack - string Cheerios on a new shoestring and let him eat them off his "necklace".
Coloring books and crayons are great if he likes to color - bring a small cup for the crayons to sit on the tray so they don't get dropped all over the place.
Get up and walk the aisles with him every so often for a few minutes as long as there isn't turbulence...no 20-month old can sit still for that long.
I hope it goes smoothly for you!!!