My dog doesn't want to pee

It's not that he can't. He simply doesn't want to. It's been raining here for a few days and he REFUSES to go outside. I have sat outside with him when it's not raining for up to an hour and he just stands there. I have sat on the wet grass with him, and he will run off the grass immediately.

He doesn't seem in pain, he's eating and drinking as normal. He just simply does not want to go out and do his business.

I know it's wet out there, but there are plenty of dry spots he could go to.

I am going to take him to the vet, but can anyone think of why he won't do this suddenly? He's going to be seven years old this year and we've had him for a year.

Update: the sun briefly came out and he eventually peed. Thank goodness.

Yes it is strange, but he is a shelter dog so I guess there are some behavioral issues to work out. It's just strange that after a year he starts acting out, but I guess also we haven't had rain like this since we got him. He is scared of storms generally, this has just been that solid, rain for days weather. No storms.