Thank you for your answer Anaïs! He's educated pretty well and behaves perfectly around people. But when there are mares coming and going in front of his stall or when your riding and mares are in the field, he gets quite "wild" and yells like crazy and completely loses focus from the job 🙈
I'm sorry, my answer might not be super useful because I don't have "tips". To me the most important thing is to educate well your horse. If he isn't educated, he won't calm down. It means that you have to really show him that you are the boss, and if he respects you, it will be way easier for you to calm his down.
Then it also depends of the context of your question : is it about calming his down when you are working in the arena along with mares ? Or is it when he is in his stall and surrounded by other mares in other stalls ? Or when he is in the field with mares ?
Sorry not to be more useful, but maybe with some context we will be able to share more tips. Have a nice day :-)