Its a long slow road to recovery from here. What he's going through is completely normal. It will be frustrating for you, you will probably feel like its your fault but its not. I don't know what weight he was to start with, but its a very common misconception "I can afford to lose a few pounds" and pick it up later. It doesn't work that way.

It is a real struggle mentally when you feel full and everything tastes bad and it hurts to swallow, you have no saliva and gag at everything. But you have to find a way.

Experiment with everything. All calories are good calories right now. Experiment with meal timings. Smaller more frequent snacks. Have a food diary, thats important when experimenting, its easy to lose track and end up eating much less than you thought.

Doing the groceries with you can be a blessing or a curse. It can be really depressing to see the range of food and feel you can't eat any of it. I had a few cries in supermarket aisles. On the other hand visual stimulation can give him ideas for new experiments. Try not to nag him, listen to him and work with him to do what he needs to do. Best of luck and keep up the good work, you've done great to get here.