My husband isn't eating enough or drinking enough. Does this mean he might be shutting down?

All his favorites are no longer what he likes. He is eating only half the food he ate a few weeks ago. I am making his favorites. He always leaves a lot on the plate. There is nothing wrong with him besides Azheimer's. He is not sick and if physically healthy, so said the Physicians' Assistant, and the bloodwork that she drew only two weeks ago said. He is acting normal in every other way for him. He is leaving stage 6 and approaching stage 7. He is completely incontinent, not knowing when he eliminates. He is still walking around and strong otherwise. His mind is gone. He doesn't talk except for a few words now. Is his body starting to shut down? Should I give him Boost or let nature take its course. He has a DNR requested long ago when he was able to request that.