My cocker spaniel puppy is nearly 4 months old and has even learned how to tell us when he wants to go outside to do his business. Imagine our concern when he unlearns every thing every time it rains. Something about the wet ground, bushes and grass puts him off. He reverts to secretively pooping in the house, on the stairs, just outside the door and so on. I am guessing he wants a hard, clean surface. Well, rainy days are going to be a part of his life so how can I get him to poop outdoors after it's rained? It took a lot of effort to retrain him after the first showers of his young life. Now it looks like rain again... ( Incidentally, I haven't started walking him yet).
I would appreciate any suggestions. Thank you.

Like many other things, it would have been easier to introduce him to walking on lead sooner. You need to do it now. It is the key to the bowel movements. Take him our and walk him. Exercise stimulates the body. While walking around, they can only hold it so long. If it is still raining, take an umbrella.

For more tips, you might read the sticky on housebreaking at https://www.askmehelpdesk.com/dogs/information-articles-our-dogs-expert-labman-53153.html#post251809

I had a lab who would rather hold it all day then go outside when it rains. We always found that taking her for a walk would get her to go. We always thought it had more to do with her not wanting to get her feet wet but who knows.