Hmm... if you're not even sure he'll be able to buckle himself, I'd be re-considering the booster idea as well. Boosters are perceived as being as easier for the kids to buckle themselves, but that's definitely not true in many (if not most) cases.
Are the buckle stalks long? Those are pretty much the only type of seatbelts I can think of that a child would have an easier time buckling than a harness. My 9 and 11 yos still struggle to buckle themselves in their boosters sometimes, particularly in our Mazda5 center row, because the buckles are very far back and low. With the backrest on the seat, this becomes even more difficult, as the child has to lean forward, past the sidewings and headwings, reach around the armrest, buckle, then adjust the seat belt properly. For comparison, my 7yo dd can buckle herself in her harnessed seats more quickly in some cases (assuming she's not dawdling, of course!)
With the Air Booster, there's the added risk that the child will miss the beltpath, so even if he does learn how to buckle himself, you'll still need to check to make sure he's done it correctly and pulled the seat belt snugly every single time - and it might be hard to see both sides of the AB beltpath without climbing in.
When we were using our Astro, I had the two boostered boys in the center row, where I could help them and check their buckling, and dd was in her Husky in the rear row, outboard by the door so I could reach back and help her if necessary, but mostly just so I could easily see her from the door. A harnessed seat is easier to reach in that position, and easier to check visually, than a booster seat.