Is he out with any other horses? I just went through this with my boy since he was out on pasture 24/7 but when out on pasture since he was sore he didn't move around much which just added to the problem. How long are you lunging him each day and how many times? He really needs to keep moving as much as possible for it to stay open and drain. Trotting is the most effective since it moves everything in that area around. Also try hosing him down there (not into the wounds just on the upper side areas and let it run down) for about 15 mins 3 times a day. My boy was so swollen by 3 weeks that I was ready to call a vet to open him back up! 4 days out with a new pasture buddy (running, bucking, playing) and the swelling was GONE! (Keep in mind he's 9 or 10 years old so this is tougher on him).
Photos would be helpful, and please keep us updated.