It is so sad. There comes a time when they get less active. I have seen that time come with so many of mine. There are times you swear they will not be there when you look in the tank the next time and then sometimes they do surprise you and are swimming like a young fish again. I wish that there were justice in this world and the beautiful fish that we all love could swim and love us for longer.
While life is not over for your fish, it does sound like he is telling you to realize that he is getting slower and I do know that as they get older the need for food actually decreases instead of increasing. So if he is getting any food at all probably he knows best at this point. I know of bettas who would not eat for days and then eat just a couple pieces of food and then go for days again. This is not the normal new
betta hungry all the time, I know but they are mature and not so aggressive. They are not eating to grow and speed about all the time, but merely to stay alive. There is a wisdom in older bettas about what they actually need.
So if he is getting any food please do not worry about him eating every day. It is better at this stage to underfeed believe me. You may not have him for long but the quality of the time you have left with him should be the best you can have. And if you can love him without being horribly worried and realize that he may just be protecting himself from feeling bad then it is not so bad.
If this sounds harsh I am sorry, it is not meant to be. I am just giving you some of what I have learned as I have known and loved my own little guys over the years. That is what I do.
I will keep you both in my thoughts and prayers.
Rose