Put the wife in the back seat with the kid. There are a lot of new sensations flying. If you can, give him a view. Even a really young baby can appreciate that. Mine got his first 737 ride at not much older than that (about 8 months), and he made a lot of nose prints on the window. It was a spectacular flight over the Rockies in winter in CAVU conditions.
Let him cruise in Mommy's lap unless there is turbulence. Use a rear facing car seat (with the FAA sticker) for takeoff, taxi and landing.
Make sure he'll wear the hearing protection at home before you try it in the airplane.
I might suggest a short and non-time-critical sightseeing flight first, so he can see the airport he took off from, maybe see some familiar landmarks, and you can go back if he gets freaked out. You know his temperament by now, so you'll have some idea if that's likely or not. But it's a good idea to test it when time is not critical.
With my own kid, I was worried about ear block. We gave him a feeding high in climb out and forgot on the descent. It was a nonissue.