Hi,
First of all big congrats to you for pitching in on behalf of your son. I really hope this plan progresses well. If you get the impression here I am speaking from experience, yes, I am.
I'd suggest he needs to feel he has a good prideful bit of kit, and a solid performance basis to talk to others about it, here online, and at astro get-togethers with you there, if communications are a bit of a challenge.
To address your questions, I think 8" is about the right size range for visual astro, and your needs right now. You need a degree of portability, respectability, and sheer aperture for urban astro and the occasional trip to a darker site. That right?
Now I've got to ask a delicate question here in public. Does your son show a liking for mechanisms and setting them up by fiddling over some time, or is his attention span shorter and better suited to a package deal, so to speak?
If the former, go for it. There is much equipment here for sale on IIS, and good quality too. We can steer you even if you buy new.
If the latter I would not get a collapsible Dob (yet). This will be contentious, and may attract differing opinions here, but I will suggest that such a Dob can be frustrating to set up and collimate properly for a young beginner. And you do have to do that, after transporting it each time and breaking it down. A collapsible Dob can supply discouragement, and I think you don't need that for your scheme. I have seen a mature bloke go to throw his 10" Lightbridge in the ocean out of sheer frustration with this setup requirement.
In
that case I would recommend a 8" Schmidt-Cassegrain scope. They are short, tubby, portable, and hold collimation quite well through being taken off the mount and put on again. There is a collimation procedure on them, but as I say, it is needed less, and is easier and more intuitive.
Alternatively you could get a 4" refractor. These don't need collimating at all, and are so easy to transport. That is about the largest size which fits in a budget If you buy the real quality brands like Takahashi, they can cost a bomb
A brand like Skywatcher is not junk, but more at the budget end.
I have not mentioned the mounting. You can get just as much controversy here, and if not selecting a Dob, then for a young beginner, I'd recommend an AZ3 with a 4" refractor, or a EQ5 equatorial mount with a 8" SCT.
Then there are the Go-To mounts. These are not a budget selection really, so maybe for later. They are motor-driven mounts which when set up properly, can slew around to any selected target in the sky. They are available in any sort of mount, and really suit the gadgeteer sort of beginning user.
Hope that assists