Eligibility to get prescription medication through the VA instead of Express Scripts.

ANY Veteran, Active Service member can get prescriptions through the VA.
Is he using a VA doctor? I am not sure but it is possible that the prescriptions the VA fills might have to come from the VA doctor.
My Husband did not have a Service Connected Disability but he got prescriptions from the VA but he went to the CBOC (Community Based Outpatient Clinic) later as he declined and I could not get him to the VA he was placed on a Home Based program and the Doctor and Nurse came to him for his Annual visit. If he had to see a specialist I would have had to get him to the VA but by then he was on Hospice so that eliminated a visit to any specialist.

This is something to discuss with the VA. I am assuming he has done so, since he tells you he cannot. Reach out to the VA through their website and ask for a "veteran's benefit advisor" through the search portion of the website. As with all entities you may wait a while for a good answer. Wishing you good luck. He should also consult doc to make certain he is getting generic scripts meds.

Express scripts is not military based. I had it for years thru BC/BS. A lot of insurance companies use them. You get 90 days of medication.
What Health insurance does he have? They may not allow a switch. I would talk this over with a VA rep at his local VA dept.