Meridith,
I am not familiar with the Law that governs your husbands incarceration and could really care less. I cannott quote the article of your U.S. constitution that would govern his position but maybe someone else here can. My U.S. colleagues have informed me that he, your husband is entitled to timely, accurate medical treatment while incarcerated.
So...
The dark spots on the x-ray are they in the lower or upper jawbone?
The fact that a tooth was extracted and a root tip was pushed into the sinus may not be relevant in regard to the dark spots in the x-ray. All upper tooth extractions bear this risk and even the most skilled dentists, ENT's and Oral surgeons have had this happen. The fact that it was not dealt with properly would cause me to be concerned with the level of care your husband is being provided. The outcomes for jaw cysts and jaw bone cancer are extremely different and the treatments are as well completely different.
In my opinion, without examining or evaluating the x-ray, for a dentist to say a patient has either cysts or bone cancer is irresponsible. Regardless of whether the patient in question is an inmate or CEO of a major coorperation dentists are bound by a code of ethics and professionalism to not discriminate against any patient for any reason and as well to properly follow-up or refer when a definitive diagnosis for any abnormaal finding is not found.
If I were in your position as a spouse I would research the law regarding medical treatment and diagnosis while incarcerated. I would also write a letter to the facility administration with your concerns re: what the dentist told your husband.
I am aware that my country and yours are very different in medical care and treatment of inmates. So, do your research.
I hope everything works out in your husbands favor.
Contact me any time if you feel I can be of assistance.
My thoughts and prayers are with you and your husband.

Mike