Or just walk him on the ground in the arena. Walk him on the ground in the arena until he gets used to it
If it's closed in where he can't get out just let him loose and let him do he's thing. &. Wait for him to calm down
Try to ride with others at home as much as possible.
Always go left to left when with others and keep an eye where other people are going and how close behind someone u r
Practicing desentesizing at home. Loud music, banners, people running around, kids screaming . That way when he sees all those horses he'll feel less stressed. Then just keep him moving . Let him look around a bit, but don't let him get super looky.
Practice at home if possible. Have others from your barn in the area all together and as others have said look where your going and be careful of the others in the ring. You will be fine :)
Talk to your horse, keep your legs as your horse will understand you're still here, as Alexia said, look where you're going to and be careful to the other riders and horses, and everything would be fine :)
Practice and practice... Riding with others was really good for me I got my head up and I was looking around and where I was going and not at the horse!