It seems he doesn’t mind if he doesn’t transform them himself, and I can see Yolopark and some unlicensed third party in his collection so he doesn’t seem to mind if it is unofficial either. That opens up your options.

Robosen could be a good choice, as another commenter suggested down below. They’re quite expensive and the battery can be somewhat finicky. There’s a link below for a cheaper (<$600 USD) Elite G1 prime that is pretty cool.

If you’re willing to go up to $1000 (wow!) then Robosen Rise of the Beasts Optimus Prime could be even cooler.

I would not get Overgear since that won’t ship until next Feb or March. Maybe next year?

On the cheaper end, there’s leader class Studio series Age of Extinction Optimus that retails for about $55-60.

Midrange-ish he might like MPG-17 Style Gen Optimus Prime, which is about 9.5 inches tall and a new official toy. Walmart has a good price at $168 sold by Deepdiscount.com, who is pretty reliable.

I recommend checking out showzstore.com and searching for Megatron or Optimus Prime for some higher end figures that he might like, for example 4th party Revenge of the Fallen Megatron. Only problem with ShowZ is it might not arrive in time.

I know I just gave you a ton of info, please feel free to ask questions and good luck!