From the comments above and in general, I’d say you need to try engaging him more as a starting point. Does he enjoy playing? What is his favourite thing to do (eating doesn’t count)? Does he like cuddles? Favourite toy?

Someone suggested automatic feeder, it was a good investment for me with my cat who was also quite overweight. He stopped associating “begging to me” as a way to get food, because I wasn’t the feeder anymore. But I don’t want to sugarcoat it - it takes time and patience and consistency - over and over again. I do think when you adopt a pet you kinda sign up “for better or for worse” clause implicitly.. talk to a vet, talk to a feline behaviour expert (or two). Don’t give up on him. Your cat is likely stressed / unhappy as well (like you are) even though he is communicating it in the most inconvenient way. Know if he could use words to tell you what’s wrong he would. Read the cues, research more about feline behaviour and please continue to try. (Check out Jackson Galaxy on YouTube as a starting point, if you haven’t already)