The 650 (twin) is my first bike. I ride it most days and every day that i do it brings a smile to my face.
As mentioned before it has many great features economy for instance - i'm only mini so if i'm not going anywhere in a hurry i get around 80mpg
It's easy to handle both in terms of cornering and picking it up after minor spills.
I have the lowered seat - the benefits of which are twofold as it is not only lower but it is shaped differently making for a greater comfort on longer rides (I can do a 500 mile day without ache) Depending on your son's height this may well be worth looking out for.
In terms of spec when shopping around must's are ABS, heated grips and centre stand (combined i think this is called the comfort pack)
There is a full on board computer version - the additions of this are a fuel gauge (the accuracy of which is subject to some debate, i don't have it so shan't comment) It also has a temperature gauge (some say the most useful feature) and a gear indicator (what can i say...)
There is also a factory lowered version out there - free when buying new i think. I'm only 5'7 and can flat foot it with just the lowered seat and not lowered suspension. Of note - the lowered suspension version reduces the pillion / load capacity by quite a bit. Can't recall figures of the top of my head.
It comes with a CZ chain that isn't great but i'm a believer in looking after your kit and it'll look after you so i've had no real worries with it. Also comes with the infamous Battlewing tyres. As i said it's my first bike, i'm not a knees down rider and have nothing to compare it with to so won't comment.
Screens are a massive topic. The stock screen is fairly useless (the stock 800 on the 650 is the same in terms of usefulness) I've found riding with no screen fitted better than these two so my search continues... People round here seem to speak most kindly of the MRA, Deserto or Britannia models. On ADV there are some followers of the mahooosive CalSci or CeeBaileys. ADV also features the Madstad screen which is similar to Deserto/Brittania. After some months of trial and error (lots of this!) I know screens are highly personal and there is no rhyme nor reason (nor science) behind riders choices.
I haven't done too much farkling otherwise - beak from the 800 (get from BMW as half the price of post market ones), crash bars (Wunderlich Adventure), LED indicators (BMW OEM), Renthal soft grips over the existing (IMO) skinny grips, Barkbusters (drop it just once and these will have paid for themselves) and all those screens!
Best of luck,
S