As hard as it is... don't make a big deal of it with him.
I agree that he is softening but if you start pointing out every slip he will withdraw again.
My H frequently at more relaxed times would use words of endearment with me because it felt natural in the moment. Later it was like we took several steps back and he acted like he never said it.
Don't say things like "yesterday you were just texting me you missed me"... big mistake. Just let it flow naturally. I know this is sooooo hard. Been there/done that.
It is a good sign. That means to keep doing what you are doing - its working.
