I have had a conversation recently about why my male puppy is humping his blankets, a lot of people said it was because he wanted to show his dominance (apparently to his blanky etc
) But I think your female could be coming into season about 5 months or shortly after, so he could be noticing this. It will be really annoying to keep seperating those 2 through their life and your almost surely to have an accidental pregnancy that way. (I am sure you aware of this) Hopefully someone else will come by and offer other advise.