My husband had the same problem. He said it felt weird and self conscious to talk to a baby that obviously doesn’t understand as if he did. What helped him was starting off with small conversations and then the complete rambling came later.
Just talk about what is going on around you at that moment. If you are watching TV, ask him “where is Pooh going?” or, “What color is Big Bird?”.
If you go on a walk say “Can you see the trees, they are green”.
If you are playing with one of his toys ask him how it works...”I didn’t know it did that”
Remember, it will be a one way conversation for awhile; listening to your words is what’s going to help him learn his words. And don’t worry that you are doing something “wrong”. There is not a right or wrong conversation that you can have with a baby.
Good Luck!