8 month old not eating enough

I am furious. My baby is 100 percent breastfed. Off the breast on demand. I have decided to go back to work and I work 8 hours. I left him formula and breast milk. When I got home only half of 1 bottle was gone. They keep saying oh he’s full he ate real food but no matter how much real food he eats he’s primarily still supposed to drink milk right? I’m afraid he’s gonna starve while I’m working and no one understands that he’s still supposed to have mostly milk even if he’s eating food. I left a variety of sippy cups and bottles and he refuses them all. Someone please help me!!!!