In the spring, your horse seems to dump copious amounts of hair all at once. Getting that loose hair off is not for the faint of heart-or your usual grooming tool. This is where the shedding tool comes in. There are many on the market, but the most commonly used are the shedding blade (usually two-sided-one side has teeth, the other is smooth) and the groomer's stone, which is a porous block (like a bot block). The stone is fairly benign but the shedding blade's sharp teeth can scratch a horse's skin and set him up for skin problems, so use shedding blades carefully and only on horses with long, thick winter coats.
Use a shedding tool on the horse each time you groom it in the spring. The horse actually begins to shed in midwinter, so you could even incorporate use of the shedding blade around January or February, depending on your climate, to aid the process. You may not get much loose hair when you first incorporate it into your grooming routine, but it will help maintain the coat and may even help with getting a shine. Don't bother obsessing over one particular spot in the coat but rather completely groom the horse at least once daily to help the horse to shed the hair more quickly. It's not realistic to think that you can get the horse to shed its entire winter coat in just one or two sessions.
Once a horse's coat starts thinning, put the shedding blade away for the season and start using rubber curry tools and lots of elbow grease. Soft rubber curries "grab" the hair and help remove it when a horse is shedding.
To convince your horse's body that it truly is spring and time to shed, be sure he has access to as much daylight as possible. The longer days of spring and summer are what trigger your horse to shed. If you keep him inside, in a barn, where it is dark or where he has shorter days, he will hold onto his hair. Also, be sure to regularly work your horse in the spring so that he breaks into a sweat. Then keep a cooler on him as he dries so he doesn't chill. Once his hair is dry, you'll find that a lot of it has released and will come off when grooming.