He will look out of place in a flying suit on his first experiences in a glider, and they are unsuitable for most initial sorties - too hot unless it is a cold day.
If you/he are really determined to get one, you can see such things as Ozee suits:
http://shop.ozee.co.uk/index
and
Ozee Exeat Flying Suit.
http://www.airstoreone.co.uk/proddet...prod=ozeeexeat.
Two words of warning: mine is difficult to open the top in flight, which you need if dressed for going high but need to be cooler low down; and it appears to have been designed by a woman, who needs no central mid-height opening but such a thing is mighty handy for a man - I had to inset a zip into mine to attend to bodily needs. Maybe they have a "male" version now.
But really, I go with the old clothes scenario as said above. Also, good clothes are magnetically attracted to dirt and oil which seem to emanate from gliding activities.
Chris N.