Veterans coming back from Iraq or Afghanistan are sometimes told that they can get extra government benefits if they have mental problems. Some of those mental problems do NOT keep a person from owning a gun, but some do. My son-in-law was a Marine, and has 100% disability for various back, leg and shoulder injuries - and brain damage. However, the brain damage he suffered is the sort that affects short term memory, and so he is still fully legal to own and use guns. Just don’t ask him which gun he shot at the range 24 hours later, or he may have forgotten...
From some of the comments I’ve read about this issue, the problem arises when a vet is deemed unable to handle their own finances. Now, whether they can or can’t actually do so, also seems to be an issue. Either way, if the VA determines you must have someone else handling your affairs, I believe you are then put on the NICS list as being incapable to buy guns. Since 2007, I believe, they now have to allow a vet the ability to petition this statement of incompetence.
God bless your son, and I thank him for his service.