Your lease horse CAN engage, it is just harder for him. His neck is not 'turned over' with the muscles in the center and top of the neck larger than the one along the jugular groove. This says he is NOT using his ring.
If you are going to continue to lease this horse, you will learn HUGELY about training if you can get him to use his ring of muscles. This horse has MUCH to teach you. Biomechanical Riding and Dressage 1
And this: http://www.equinestudies.org/ring_re...s_2008_pdf.pdf