My son is a PFC stationed in S. Korea. He is 19 and has been…

A military member has to be what's generally called fit for full duty in a worldwide assignment situation.
Based on this issue it would seem that he's temporarily unfit for full duty due to his illness, surgery, and potential need for additional surgery.
Should they decide that he's not medically qualified they will determine whether he should be separated for medical reasons. There is a medical evaluation process for that.
Should they determine he should be medically discharged it's possible he will qualify for some percentage of disability rating. That may entitle him to further treatment and VA benefits. Those benefits are especially important if the illness is caused by or aggravated by a condition of service.
A medical discharge is "honorable," and he should qualify for education benefits so long as he has served at least two years and receives and honorable discharge; or

Post – 9/11 GI Bill

Eligibility: The Post- 9/11 GI Bill is a new education benefit program for servicemembers and veterans who served on active duty on or after Sept.11, 2001. Benefits are payable for training pursued on or after Aug. 1, 2009. No payments can be made under this program for training pursued before that date.

To be eligible, the servicemember or veteran must serve at least 90 aggregate days on active duty after Sept. 10, 2001, and remain on active duty or be honorably:

1. Discharged from active duty status;
2. Released from active duty and placed on the retired list or temporary disability retired list;
3. Released from active duty and transferred to the Fleet Reserve or Fleet Marine Corps Reserve;
4. Released from active duty for further service in a reserve component of the Armed Forces.

Veterans may also be eligible if they were honorably discharged from active duty for a service-connected disability after serving 30 continuous days after Sept. 10, 2001.

Depending on how the VA rates him (typically at least 30%) he may get additional benefits.
Here is a link to the VA benefits. And here are his medical benefits, depending on how they view his illness.

Basic Eligibility

A person who served in the active military, naval, or air service and who was discharged or released under conditions other than dishonorable may qualify for VA health care benefits. Reservists and National Guard members may also qualify for VA health care benefits if they were called to active duty (other than for training only) by a Federal order and completed the full period for which they were called or ordered to active duty.

Minimum Duty Requirements: Veterans who enlisted after Sept. 7, 1980, or who entered active duty after Oct. 16, 1981, must have served 24 continuous months or the full period for which they were called to active duty in order to be eligible. This minimum duty requirement may not apply to veterans discharged for hardship, early out or a disability incurred or aggravated in the line of duty.

 

Unfortunately it is hard to guage his exact rights at the moment until they have finished evaluating his surgery and a prognosis for return to full duty.

Holding companies or temporary assignments waiting for medical care do present difficult situations, I understand.

Sincerely.