Testing them in arcades won't be so easy: noisy environment, mostly recent games. But you can still show pictures / videos on youtube or pinball simulations to know if he likes the pin or not.

Many suggestions above seem good.
- Bobby Orr's power play is a great game.
- Black hole fits your bill but is indeed unforgiving: you may hear "failed reentry attempt" too much. I wonder if one can adjust settings to avoid losing the ball after playing on the lower playfield.
- Genie is indeed very slow...
- World Poker Tour seems a good suggestion for a recent game

Among the machine I own, three would fit your agenda (open playfield, limited or no multiball, ball easy to track, not too fast (?)):
- Banzai Run: Can be fast, but there is a nice large circular ramp. He may enjoy the vertical PF as well. Limited multiball.
- Bally Spectrum: lots of drop targets, 3 balls but never more than one played at a time (!) But the side lanes are hidden, so you can't keep track of the ball all the time.
- Zaccaria Pinball Champ. Most 2nd generation zac would fit your bill: no multiball, interesting shots, transparent upper playfield allowing you to follow the ball at all times.

Besides that, widebodies from the 80's such as Paragon, Volcano, Devil's dare and space invaders (already cited) could be considered.

Hope this helps,

Good for you! Welcome aboard, and I hope your son will enjoy this Pin as well. As a side benefit, you may enjoy it as well.

There are good guys on that forum. Glad you found one.