I had migraines almost every day for 2 weeks before my lymph node swelled up. Doctors did not have any explanation for the migraines, saying that it was possibly unrelated or due to stress. I wasn't stressed, so it wasn't that. In my case, I get migraines with auras, which your husband may not, but still, headaches. Also, the migraines went away after the tonsillectomy and neck dissection. I have had 2 in the past year whereas I used to get 1 or 2 per month before treatment. So, for me, there is a correlation. Doctors may not understand why it is or even acknowledge it, but they don't know everything about everything. My radiologist was the only one to say that it could possibly be related, saying that "we don't fully know how the body is integrated and connected." He said the headaches could have been caused by the cancer. Until our understanding of medicine is farther along, we can't know for sure. I appreciated having a doctor be honest and say "I don't know." It was also nice to hear "I believe you and don't think you are creating it through stress or imagination."
Good luck!!