It's normal for all horses to lay down in a place they are comfortable. They will stay down anywhere from a couple of minutes to about 45 minutes. If you're worried go ask him to get up. If he doesn't want to get up, doesn't shake after getting up or goes right back down, then it's not normal
All horses love laying down! But if they refuse to get up, this may be a sign of a deadly sickness called colic! If you suspect colic, call the vet immediately and walk your horse in small circles until the vet arrives.