as US citizen you have to apply for visa before arriving in Vietnam which gives you 90 days. Vietnam doesn't do extension so you have to leave VN after 90 days
so technically you aren't retired from the military either. You're just honorable discharge and receive VA benefits due to your health conditions which means you only have the VA as health insurance and for overseas you have to enroll into VAFMP but as I've said it's only for service connected conditions and it takes forever to get reimbursement according to many vets.
Investment visa not many recommend this for long stay because it's a lot to handle and you only can own 49% of the company and 51% has to be owned by a Thai.
Visa run I was told by many that I could apply for METV to get in which gives me 60 days after that I have to request for extension which gives me another 30 days. After those 90 days I have to leave thailand to go to the neighbor countries and apply for METV again to repeat the process but I would ask someone specifically who has done that because Thailand changes visa rules like changing underwear 😂
if he marries a Thai then surely he can get marriage visa.
Visa run surely works if he applies for METV that allows him to stay 60 days then request extension for 30 days. After those 90 days he leaves Thai to Vietnam or wherever and repeats. I've never done it but read many have and was recommended to do that