First, document those missed/cancelled appointments and ask to have the records changed. If that’s not possible, you can also ask for a letter you write to be included in his medical file. Keep a copy for yourself. You can also ask for a change in psychologist, or you can go to a Vet Center which has walk-in mental healthcare if there’s one near you. A change in psychologist is possible if there are others at the facility and ask for the paperwork to begin that process.