4 month old nephew

So looking for suggestions... I watch my 4 month old nephew 2 days a week. He is breastfed. She went back to work about a month ago so I've been watching him for about a month now. First few times were good. He was feeding fine. Only cat napping but was in good moods so that's fine. The last few times I've watched him he will not eat for me. He screams bloody murder for hours. He's with his grandma 3 days a week and she's having the same problem. He's on Zantac already. My 6 month old has a milk allergy and we found out due to mucus in his stool. My nephews looks exactly the same. She said she had the doc check it at his appt right before thankgiving and it was fine. Last time I watched him he ate 1 oz between 4am and 2:45 pm. I keep bringing it up to my sister in law and she pretty much just shrugs and has no ideas for me. I've tried walking while I feed, extra burping, heating the milk warmer, not heating as much. I feel bad for letting him scream but I have a 6 month old too I have to take care of. I figured the first couple weeks would be rough but it's going on 6 weeks and she works full time so he's with me or gma all week. I guess I just figured he would be getting used to it by now. And now I'm parking my 6 month old in front of the tv all the time so I can take care of the other one. I'm just out of ideas and I literally dread babysitting days 😕