I had this exact thing happen to me last year with one of my colts, and I had no vet to call, so I was even more panic-stricken because of that. My colt was OK, and it became unblocked...not sure if on it's own or from something I did, but I just tried kind of hanging him upside down, also massaged the throat right from the head on down the neck. I could not feel the blockage, but jhoped that my pushing around on the pipe, maybe I could somehow change the cshape of the poop and get it to pass. I also put some water in his mouth from time o time, hoping that might also help break it up. Not alot, and I never forced it in with a syring or anything, just cupped it with my hand. I was so scared, but thankfully, in the end he was OK! It went on for at least an hour.
Good Luck with your colt! I know how scary it is!