Hey Fedup,
The reason they change his meds is because of the bodies way of adapting itself to the medication, thus making it less effective, and you basically end up with high or excessive doses to do the original job it did when first prescribed. Changing meds helps the symptoms by giving the body something new, something it has not adapted too, thus the medication takes effect all over again. Medication has a 8 week cycle in withdrawals to come off one, and another 8 weeks for the new one to take effect. Approximate figures for general SSRI's.
You said, "I don't see any improvement" above, and that leads me to think that your thought perception on medication is that it will cure your husband, or make him better!!! It is medically documented, that no medication will actually cease symptoms of PTSD, in fact, may make some worse than untreated. Medication does not, will not, and can not stop the symptoms, it only takes the edge of it working correctly within the individual.
The only way to effectively control the symptoms of PTSD is through active education, learning, trauma therapy, exercise, diet, etc etc. They are the only known means at this time to really sustain and control the symptoms of PTSD. Methods are available, such as EMDR, but this is still not a cure, just a method to invoke trauma therapy. With PTSD, we have trauma, depression, anxiety, social withdrawals, hypervigilence, etc etc etc... the list is long. Medication will only take the edge off some symptoms, or may provoke others to be more prevalent, EMDR is a form of faster and more accurate trauma therapy, CBT is a form of learning how to control your symptoms and structure your life to handle and cope with PTSD as part of your everyday life.
So many things, none of which 100% work, as PTSD has no cure as yet... though there are people working on it, and some believe they may off already found one, as a specific technique has worked for them with no symptoms caused. So much information...
Your husband needs to begin education on PTSD itself, and learn how to live with it, instead of all the trouble it causes you both. It is harder to get older people to move with new therapies, because they are often stuck in their ways living with it for 20+ years, but as they should be able to see... their methods aren't working, so to try something new could only be a good thing for them.