Hi Koas house, and welcome to Pet Talk!
I read on one of your posts where you don't let the puppy defecate on the grass. This may be causing some confusion for him.
I'd allow the puppy to go on the grass and get used to going outside the house vs. inside the house first, then worry about if he goes on the grass or the pea gravel later.
Don't panic. He's still young and he can learn all of these things in time, with positive re-enforecent and repitition.
(Sometimes they will temporarily unlearn something they've already learned by being brought into a new environment or just from being distracted in some way.)
Just a thought.
Congrats on your new baby!