I breastfed all the my girls. Crystal is 2 and Calleigh is 16weeks. Crystal we breastfed right to the 12m mark, Calleigh I was forced to stop because she has a complete milk protein allergy and soy allergy.
Unless I put her on formula for 1 week and eliminated everything under the sun (literally fresh meat and fruits/vegs only no seasonings etc) then placed her back on breastmilk I couldn't do it. I didn't want to shock her system by going off the BM then on formula then back on breastmilk to be taking off again on a opps. So we went to Alimentum and have been there. Calleigh is 10weeks early and its extremely common for preemies to have milk allergies
A GI tested her.... I GI is a wealth of information.
None of the drugs for reflux wil help with spitting up amounts. They are to reduce the acid content so that it doesn't harm the child and hurt them.
Calleigh is on prevacid and still spits up constantly.
Im no doctor but it just sounds like a weight issue.
Try mixing 3/4 scoops of the formula with 2 ounces of water, it will give them extra calories. It could be that your BM is lacking the fat content he needs, maybe big brother is scuking it all away you never know. I would nurse then give him at least 2/4ounces of formula and see how he does with the weight issue.
If he is spitting up you can do rice in the bottles but unless he is spitting up ounces upon ounces I never found it to do anything but make a mess when it came back up