High magnesium feeds are ment to be good for calming
Consistently and a low starch/sugar diet can really help
Changing to a cool energy feed program, speak to a nutritionist.
Calming supplement such as Omega Alpha Chill. Many others on the market.
A consistent workout program for the horse 4-6 days a week. Consider lunging in the morning and another 15 minutes before your actual ride.
Supplements, consider changing his feed
You can use calming supplements. A bunch of different brands carry them