I would not put him in a Short plate.
I had a female Intro to Tech student last week that is only slightly taller than 5'. I put her in a Short plate for the first day. The top of the plate was hitting her in a place on her shoulder blades (out of the water) that was causing discomfort.
I put her in a standard sized plate the next day and she was fine. And a standard sized plate was certainly not too tall or wide.
I also would not buy a back plate that had no slots for cam bands (thus requiring you to always use an STA), as I believe the Halcyon Short plates are made. I have never used an STA and would not care to. I like my cylinder to be as close to my body as possible. An STA just holds it up higher off the bank. STAs are simply a band-aid for having a BP that does not have slots for cam bands - as they used to be in the old days.
Similarly, I would not buy a wing that does not have slots for cam bands, nor a wing that only had one set of holes for placement on the back plate. Having slots for cam bands does not mean you can't use an STA if you want to. It just means you CAN ditch the STA, if you want to.
There is no downside to having cam band slots in a wing and BP. There is only a downside to NOT having those slots.
Wings with only one set of holes for attaching to a BP are similarly limited in their flexibility. Sometimes, you might actually WANT to position the wing higher or lower on the BP. Most wings have at least two sets of holes for that. I really don't even understand why Halcyon insists on continuing to make wings with only one set of holes and no cam band slots. But, they do, and there are numerous other options that are JUST as good, do have those extra features, and are less money.
Aluminum plates are fine for single tank. But, when you use them for doubles, the bolts eventually wallow out the bolt holes in the back plate. For that reason, if I wanted a lightweight plate, I would get one like the Dive Rite XT Lite, which is stainless steel, but skeletonized to make it lighter weight. Or scour the used market for a DSS kydex backplate (which is actually what I use for most diving that I do in doubles). But, if you know you are never going to use it for diving doubles, then AL is perfectly fine (in my opinion).