some of the newer toys can be a bit tricky. I'll second the recommendation of the Classics Deluxe 2-Pack Prime. The truck mode isn't the best, but he transforms well and a cool gun. Sculpting is nice and plenty of articulation. I think the worst thing about him is the size. If he were Voyager he'd be my main Optimus.
The Megatron he comes with is pretty fun too. He's got a spinning gimmick that should appeal to kids (it appealed to the big kid in me) and is a pretty decent tank. Not very G1 though, sadly.
Cyberverse is a good suggestion too, as there are a few vehicles/playsets out, and some of the toys are really fun. Cliffjumper from the TF Prime line is highly recommended. Going back there are some G1 styled Legends, including a little gun Megatron and there are all sorts of movie, Prime and G1 styled Optimus Primes in that scale.
Another line to think about might be the Botshots series. There's loads of different characters, and the transformations are very simple (I don't have any but they appear to auto morph using a pull back and go motor thing). Characters seem to be based on a mix of G1 and Movie casts and designs, so could allow some imaginative play for a child.
If he likes building blocks, like LEGO (but not too like LEGO to be legally actionable) there are the Kre-o sets. Mostly G1 type designs with some movie stuff in there too, but branching into Prime with the Beast Hunters series, each kit comes with a mini-figure or two and these are sold separately too for a small price, maybe giving your son something to save up pocket money for, if that's still a thing?
assembling a Neo-G1/TF:TM cast. Please PM if you have (or know of) the following at a reasonable price: Classics or Henkei Prowl, Astrotrain, Ramjet, Skywarp, Sunstreaker, Sideswipe, Blurr, Perceptor, GDO Wheelie and Springer, or 3rd Party Motobot Arcee, FP Springer, iGear Ratchet and Ironhide, City Commander (and/or Ultra Magnus), Starscream (deluxe) Coronation add-on, or Spike and Daniel in their Exo-Suits.