Switching from expressed breastmilk to formula

My son is 5.5 months old and has always been difficult to feed. I tried breastfeeding for 3 months but gave up and decided to exclusively pump and feed him expressed breast milk (EBM). He transitioned to the bottle fairly easily but has always been fussy - eating small portions (much less than the amount recommended for his size), screaming through feeds etc. Over the past 2 months he has been doing much better but now I am trying to make the switch from EBM to formula and am having problems. The first attempt was a complete failure and he ended up refusing everything. He had a nap and woke up hungry so I offered the formula and he ate it heartily. The next feed he ate about 1/2 his bottle of formula and refused the rest. The next feed (4.5 hours later) he again refused the formula and then wouldn't take anything. He is already very small (3rd percentile) and I am concerned about what to do. I am not sure if I should continue to offer only formula or switch back and forth. I also have a freezer full of EBM (400 oz) that I would like to use but I want to make sure he will take formula before I run out of EBM. What is the best way to transition? And at what point should I be concerned that he's not eating enough? Today he ate only 15 oz compared to the 25-30 he should have.

[Renee Radenberg]

I recommend to transition from breast milk to formula is to mix the two. Start with ¾ breast milk and ¼ formula, go to half and half, then ¼ breast milk and ¾ formula, and finally to all formula. You can do each mix every 3-4 days as long as he is accepting the mixture. This way you will use up the breast milk and he will get used to the formula taste. Once you are on formula and he is accepting it, do not reintroduce breast milk bottles. If you still have breast milk leftover, use it in his rice cereal (if he is on the cereal). I suggest you have him checked by his MD because you stated he “screams through the feedings”. Something medically maybe going on. Best of luck.