Okay, this will probably sound cheesy, but since you don't know what size to make it, maybe you can make something that attaches to the strap he's using now... sort of like a seat-belt cover, KWIM?
Hmm... do you know how tall he is? DH is a little over 6'2" -- I could measure one of his for you, if you want, though I'm not sure people always hold their guitars the same way. Might give you a decent guesstimate, though.
Anyway, if you haven't come across it already, here's a knitty pattern
for a girly, non-adjustable guitar strap. I know it's not what you want, but it may be helpful as far as explaining how to reinforce it to keep it from stretching.