When I bought my colt at eight months old I had the same situation- he had grown his winter coat before shedding his foal coat and it was extremely thick. When spring came I just groomed him daily and although it took a long time to shed out completely I let nature take it's course. I did break down and took scissors to the long hairs under his jaw so you could see his beautiful head, but other than that I left it alone. When he did finish shedding he had a beautiful shiny coat, due to all the grooming he had. But also, have you wormed him or had him checked for worms? Sometimes if they are wormy their coats will shed more slowly, I have heard.