Believe it or not they are rarer new or used than the 1150; hold their value as well if not better; and are in many ways a more "sensible" buy than the 1150.
Much lower insurance (Group 8)
Fantastic fuel economy (60mpg to 80mpg)
Cheaper servicing costs
Cheaper to buy (£4995 new)
Very good 2nd hand market for sellers but largely throught dealerships (buyers tend to buy from BMW).
Parts, spares, accessories are all cheaper than 1150 bits (e.g., engine bars £65).
The 650GS can be customised as much as the 1150: larger fuel tank, remus/laser exhaust, techlusion chip, bespoke luggage, bush guard, handguards, aftermarket screens, bar risers, blah blah blah.
I am 6'4" and 18 stone in my kit and I regularly ride my girlfriend's 650GS and love every minute of it. It is small, however, and if I was to have one myself I'd get the Dakar version with the raised ride height and larger front wheel.
Shod with Continental Twinduros the 650 is a much better off-road bike than the 1100/1150. It's much lighter and the gearing suits off-road better. On road, a 650 ridden hard will keep up with an 1150 up until about 80-90mph. It runs out of puff thereafter and your son will struggle to see much over a ton with his size and weight. The 650 will tour without any trouble providing you're not looking to eat meg-miles at 90+ on motorways - it's most at home on fast B-roads and twisty A-roads. It'll handle itself surprisingly well and surprise a few sportsbikes, yet it's a fantastic first bike - easy to ride and very forgiving.
The 650 doesn't benefit from paralever front suspension as does the 1100/1150 so I'd be more inclined to get ABS (it's a cheaper option for the 650 than the 1150). A hard pull on the front brake can lock the wheel more easily and unexpectedly than on the 1150.
The 650 needs a screen (it comes without one) but the BMW clear touring screen is crap. Buy the black opaque Dakar screen or an aftermarket jobbie such as the Wunderlich touring screen. Get heated grips and handguards. BMW luggage for the GS is pukka, much better than that for the 1150, and costs less.
Although your son is on the big side for the 650, if he can find a Dakar version (I know where there's a nice Black & White one) he'll be quite comfortable and it'll handle his weight. It's the perfect start for somebody looking for GS fun at a cheaper price than the 1150 - although you can get 1100GSs for the same money the insurance and running costs would be much higher.