Baby not eating enough

Really just here to vent I guess. My 4 month old has fallen off his growth curve and is now only in 4% for weight. He's so distracted while nursing that I'm physically wrestling him trying to get him to eat. I've also been trying to give him bottles as well but so far he hates all the ones I've tried. Or he hates expressed milk. Or he hates formula. Idk but I'm so stressed thinking of him not eating enough and it's not like I'm not trying to give him more food. He seems like a happy baby and content with less food, which is even more sad to think about. I'm feeling so helpless.