You could always take him to the LBS because you "want to check out some stuff" for your bike, and while you guys are there, try out some bikes together. Just for "fun" I mean, who goes to a bike store and doesn't eyeball and maybe try out the new shinies? That would at least give you a better idea of what fits him and what he finds comfortable and it probably wouldn't tip him off that you are looking for something for him.