Need help understanding my 9 week old

Can someone help me understand why my 9 week old is drinking less but prefers to just sleep more? He doesn’t even finish 1 and a half ounces sometimes. I breastfeed and give formula milk both. He will nurse from me a short while too only. A bit worried that he may not be drinking enough. He also can’t drink a full 3 oz or 4 oz now at 9 weeks :3