My senior pug won't eat, is losing weight, and sleeps all the time.

Hello and thank you for reaching out to Petco Pet Education Center, formerly Petcoach. I'm sorry to hear that Gum seems to have a decreased appetite and is more lethargic. While these are vague clinical signs, these may be due to some sort of underlying disease of either the gastrointestinal tract, liver or kidney disease, and/or a possible neoplastic or cancerous process. Since he has had a decreased appetite long enough to lose weight, I recommend having him evaluated by your veterinarian at their next available appointment. Your vet can perform bloodwork to look for any evidence of systemic illness that may be contributing to his clinical signs. His clinical signs are unlikely due to behavioral changes alone. I hope this is helpful - best of luck with Gum!