Hi.my Baby Boy Was Born 7th Of May. He Was

Hi.my baby boy was born 7th of may. He was burned 2.8 kilos, but, because I had difficult delivery (vasa previa) we left the hospital after 10 days with 2.5 kilos. I breastfeed the baby for the first two months but the baby was vomiting a lot so I had to include formula milk (aptamil). At the end of the second month he was 3,6 kilos. At the end of the third month he is 5 kilos, 60 cm long. How can I improve his weight because he is still vomiting and I have to keep him at 45 degrees to keep that under control? Thank you in advance