How about a reward system? If you are at your Mom's, and Stewie pees in the RIGHT place, reward him with a treat he likes. We use a small bin (like a cat litter bin) and put the pee pad in that. It seems to work well, and we just layer the pads when one is used. Although we do leave it outside, so I can't vouch for the smell of it. If we are gone for extended period, we do bring in his pee pad and bin inside.
I think it just takes time (yes, yes, I always say that) and if he's not there all the time, he may not be "used" to it. I think he'll get it sooner or later, he looks like a smart pup.
