Baby won’t eat

My 5 week old son just isn’t eating as much as he was. When he was in the NICU I was able to pump and he was taking 100ml of my breast milk or similac advance if I ran out every 4 hours. When we got home I could no longer pump so I started giving him formula but switched to gerber soothe because advance was killing his tummy. After a few pediatrician appointments he now has rice cereal added to every bottle as well as Karo syrup, Zantac and vitamin d twice a day. Now we’re lucky to get him to take 60ml every feeding but it’s usually closer to 30-40ml. Tonight from 6pm-10pm he took 180ml but it’s 4am and he’s refused to eat since then.

I’m worried somethings wrong but I wonder if he’s just getting fuller easier with formula and rice cereal than he was with breast milk. We will fix 60ml and he will take a few swigs then just go back to sleep or spit the bottle out. He’s been very constipated since we got home which I expect because of what he’s eating but even after he’s pooped I can barely get him to finish his bottle. He doesn’t really spit up and we’re still getting plenty of wet diapers. It’s just hard to make bottle after bottle and him only take a sip out of it or barely finish half.