The responses are concerned with 'when walking' but let us not forget the nights in the refugios ... as a first-timer he most likely won't know what is expected in these gender mixed hostels. Even in hot weather it is not considered appropriate to walk around indoors in just t-shirts and shorts (or, heaven forbid, just underwear) - this is Catholic Spain and it is a Pilgrimage ... no, men - and foreign gentlemen in particular - are expected to dress modestly at all times.
Minimum requirements are a pair of plain flannel pyjamas for bed wear and a tartan dressing gown for moving around the hostel.
Some innovative gentleman are taking to wearing silk pyjamas to save weight but this is quite frowned upon in the more traditional areas of Spain! :shock:
As I have mentioned elsewhere last year, although the clothing is expected to be plain - this is a serious pilgrimage after all - one is allowed to express some individuality in choice of footwear (slippers are expected to be worn at all times) and moroccan tooled leather slip-ons were quite the thing a year or so ago.
finally, to do with the vexed bed-bug problem - a new type of pyjamas has appeared. These do seem to offer some protection but are so figure fitting that they are quite immodest. If you choose these you must keep your dressing gown done up at all times!
A bit of extra weight of course, but if it does become cold you can wear the dressing gown whilst walking along - very elegant! - some pics below.
I do hope that this helps :wink: