Liamsnanna - I'd suggest you stick with nappy pins for now as they work for you. I've seen these Nappi Nippas but not sure I'd trust them for an older child.
With regard to the training issue, from your other thread, I appreciate you don't want to cause any stress in his life for now, and others have suggested leaving it till warmer weather. However, having a potty in bathroom, and leaving nappy off for a short time after a bath won't cause any stress and he might just give the potty a try. My boys often used to pee as soon as I put their feet in the water (some kind of reflex reaction - helped by sound of running tap) and I would grap the potty and hold it in front of them. It didn't mean they got trained immediately, but at least gave them the idea.
Well done with your care for grandson 