Hi and welcome to IIS.
I'm sure you'll get plenty of opinions, but mine is that an 8" manual/"push-to" Dobsonian is a good choice for a 12 yo:
- Simple and relatively unbreakable
- Quick to setup, nothing to learn (except the stars) - suited to the short attention span of a 12yo
- Good value-for-money
- Easy to sell - there're always other begineers looking for a Dobsonian
- 8" is not too heavy - your son can probably lift it himself
- Few if any accessories needed (save eyepieces and maybe star charts and a red torch)
- Will last for years without anything wearing out
- Enough aperture to grow with him and keep him interested for years
You can get an 8" Dobsonian (or Dob for short) in either solid tube or collapsible "truss tube" styles. For 8", I don't see any need to pay extra for the truss tube, so I'd go with the solid tube style.
There are three brands of Dobsonian easily available: Skywatcher, Saxon and GSO, In my opinion, it's much of a muchness - Skywatcher/Saxon are virtually identical and GSO not much different. Pick whichever one you like.
You'll get a couple of eyepieces and these will do to start with, but after a while, you/he may want to get slightly better/more eyepieces - that's another discussion entirely and not that important right now.
There's free planetarium software you can download - Stellarium - and it's easy to use - well suited to a beginner.
For outside use, a book of star charts and a cheap red torch (or a torch with a few layers of red cellophane over it) will also be useful. There are plenty of books he can also read suited to beginning astronomy, but I'll leave that to others as I don't have any myself.
Hope this helps.