This works very well for us: When your pup first wakes up or straight after a meal he will want to pee. Take him straight out and give him a command and praise and maybe give him a treat when he does what he cannot help doing anyway. He will soon associate the verbal command with peeing and getting a treat and praise. You can reduce the times he actually gets a treat quite quickly. When you are just about to open the door start saying the command and it should re-inforce the going outside thing. Then when it is raining he knows exactly what you want. We find this works. We tell Idgie to 'do your business'. Suggest you pick something not too embassing if overheard and something not likley to be overhead in normal conversation by your pup indoors! If your puppy learns to do his business on command, he will know what you want done promptly when it is peeing from the sky! We find it pretty convenient all round, especially when it is raining cats and dogs. I get her to do #2s in our own garden before her walks, to save carrying around a full bag! She will do both # 1s and # 2s to the same command, which ever she needs to do most. Of course your boy will still torment you later on by saving a bit for each mail post!
Reminds me of the great confusious saying - never pick-pocket a dog-walker!
Good Luck.
Tony