He'll 90% be fine with his normal size. Even TW's sizing info says that they fit the same. Every foot is different (and like you for some ppl sizing will be different), but no need to worry him after he's ordered as it'll most likely be an 8.5 for him in both shoes. (I also own GR5s and V9s).
BTW, Peter, out of curiosity which shoe do you prefer between the GR5 and V9? They are both great shoes for non-HC use, they feel like running shoes but with much better lateral stability.