Prisons are notorious for being unresponsive when it comes to providing adequate medical treatment, and depending upon the seriousness of the condition, this can rise to the level of a civil rights violation.
An inmate has the right to adequate medical care. The Supreme Court has held that a lack of adequate medical care is a form of cruel and unusual punishment, which makes it a violation of the 8th Amendment to the Constitution.
The prison doesn't have to provide an inmate with the same caliber of treatment that he can get on the outside. But prisons are not allowed to be indifferent and turn a blind eye to a serious medical problem. You can get an idea of what courts have found to be deliberate indifference and what sorts of situations constitute a serious medical need here in this PDF I am attaching.
It has been my experience that when an inmate needs medical treatment and isn't getting it, that family and friends aren't usually able to accomplish much by calling or emailing the prison. Generally, the authorities just give family the runaround and they won't address inmate care issues with them.
Contact your son's lawyer and have him or her look into the matter. Lawyers can see an inmate without having to wait until visiting hours or days, which is more than what anyone else can do. They can also see the shape the inmate is in. They can contact the prison's legal team and put them on notice of a civil rights violation and get them to look into treatment. They can also get the case back in front of the judge, who can order medical treatment.
If his lawyer can't/won't do anything, another step you could take is to contact the American Civil Liberties Union. The ACLU been advocating for inmates rights all over the country, and even if it turns out that they can't take on your son's specific case, they may be able to steer you to someone local who can.
The ACLU is at ACLU.org. They have branches throughout the United States, and this sort of a situation is one that's right up their alley. If you cannot afford a lawyer for your son, the ACLU may even know of local lawyers who would take on his matter pro bono.