It takes time but am sure the weather hasn't helped. We had great weather when we got Darcey which meant the door was alsways open and you're more patient to stand there, but I have to take her out on a lead if it's raining. Then she knows she's got to go before she goes in!
You might be right, he hates the rain! Bless him he gets so shivery.
It's funny isn't it when it's just been a few days but feels like much longer!