Oh, I feel your pain, Mary! None of our dogs likes to be wet, so it can be a struggle to get them to go. About the only thing you can do is stick with it and eventually, your consistency will pay off.
Not sure how much dog experience you have, so I'll just start with basic suggestions.
Buy a good raincoat if you don't have one!
But seriously, when it's time for a potty trip, your patience will last much longer if you're comfortable out there. You want to make sure you can out-wait your fella. He'll figure out that the faster he goes, the faster he gets to go back inside where it's dry.
Is he housebroken on dry days or is this his first experience with elimination training? If this is his first experience, then treat him just like a pup. Take him out frequently and don't forget to praise him when he goes outside.
As for teaching him to go on command, does he know, yet, what the command means? Timing is everything--if you give the command and then wait for the act, it'll take a lot longer than if you wait for him to go by himself, and repeat your command while he's in the act. Of course, in the early days, waiting for the act is more frustrating for the humans, since we'd like them to just go and get it over with, already! But it's easier for them to grasp the command if they hear it at the right moment.
What's your pup's name? I hope you share some pics with us when you have time!
And welcome to the board!