Bring activities. Airplane flights can be a great time to work on crafts and practice writing numbers and letters. Make sure you bring along plenty of activities (remember the attention span of a preschooler is about 15-20min!). Also bring their favorite blanket or stuffed animal to help them take a nap. Bring some easy to eat finger food snacks such as cheerios, dried fruit, cut up veggies, etc. Make it fun!
Colds/allergies? During take-off and landing, cabin pressure changes can cause sinus or ear-pain in children and adults with rhinitis or sinusitis due to colds/ allergies. Hydration, a bottle or pacifier (helps with suctioning if the toddler still uses them), or a dose of Acetaminophen might help for those 20-30 minutes sometimes embarassing of the flight. Talk to your doctor for advice if your child have symptoms.
Carseat. I would bring your carseat on the plane and strap him in as if you were going on a long car trip. This will bring some familiarity to him so he won't feel like he needs to jump out and run around the aisles. Also have him suck on something (lollipop) on take-off and landing to help equalize the pressure in his ears and avoid pain.
Infant earache. Taking a young child (under 3 but more significantly under 1 y/o) on a plane can become stressful, particularly when on descent - the connection between the ear and the throat is soft and easily collapses. Babies don't know that yawning will "pop their ears". Strategies include using gum in older children (monitor for safety)or having them have a drink (or their bottle) when descent begins.
Keep occupied. Keep him occupied. Discuss the trip in advance and use time on the plane to learn about the place you are going to. On the way back draw a pictures from your visit.