So mad and frustrated, need any potential advice

So I took my 7 week old son to stay with his father for the first time on Sunday evening, until tomorrow morning. I packed him enough pumped milk for the stay, plus a few extra meals just in case. His mom keeps him for both of us while we work during the day. She told me Saturday that I needed to pack more food for him because he was always still hungry, but I'm already packing him 12 oz for and 8 hour period which I think should be more than enough if she's feeding him properly because I don't have any issues with him acting hungry when I have him. Maybe I'm wrong? But when I dropped him off, I gave him the rundown of feeding him every 3 hours during the day, and supplementing with formula at night to help him sleep longer. I also told him to make sure his mom wasn't overfeeding him while she watched him. Then he texted me at 3 this afternoon saying he only had two meals left. Wtf?! I don't even understand how they've gone through so much milk so quickly, but I'm barely pumping enough as is to be able to stockpile. I feel like I'm going to be forced to switch him to formula because they're feeding him more than he needs. I'm just frustrated. Any thoughts or advice would be appreciated.