You're braiding it too tight. The closer to the crest it is, the tighter it is. Have you let him put his neck down with the braid in to test it? If it pulls taught, it's too tight and causing him discomfort. Braid as far away from the crest as possible then when you're done, pull it upwards to loosen the braid even more. I love doing running braids on my colt. His mane has just now gotten long enough to do them, but it's not yet long enough to leave in overnight.
There on the tips of fair fresh flowers feedeth he; How joyous his neigh,
there in the midst of sacred pollen hidden, all hidden he; how joyous his neigh