thanks for the answers ! 

the stallion has left now, but my horse seems fine with it.  thanks !!

I know that this reply is a bit late, but if you find that after bonding with him that he is still upset, then rose oil can help to ease emotional trauma and loss. It's a completely natural rememedy so it's safe to use and it can be easily bought for a low price from your local pharmacy. If you've never used natural rememdies before, then you need to blend the rose oil with a macerated base oil to avoid irritating him. Offer the rose oil to him and he will tell whether he wants to take by inhalation, orally, topically or not at all. Hope this helps, and good luck!

You are his best friend too by the way! Make him feel better by giving him occasional treats and spending quality time with him. Talk to him and stroke him, he'll feel a lot better!

Just keep on bonding with him so he knows you care and understand him. Try letting him spend some time with other horses so he can have more friends (or maybe get another best friend!).

At first it will be hard for your horse, but if you bond with him more (which, i know, you probably already have) he can relax and start getting used to the fact that he won't have his best friend around anymore.

I hope this helps!