I wouldn't switch him directly over to formula all at once, it may cause further digestive problems including gas and further constipation. When I switched my 1st child from breastmilk to formula. As per my lactation consultants advice (my son had problems latching) I pumped my milk so that I could make a mixture of breastmilk and formula. 75% breastmilk and 25% formula for several days, then 50/50 for a few more days and so on. It really helped him with his digestive system and he took to it a lot better. Also, you can try a different nipple because baby may not like the nipple your using. I also agree with the post above^^put some beatnik on the tip to coax baby to drink. Good luck! :)
Hi when i stopted breast feed and she didnt want the bottle i would put a bite breast milk on the tip and hold here like i was breast feeding and after a hard week she star eating bottle like crazy , when i try formula my baby got constipatet from it at the beging so a nurse told me to give here the inside of a cherry tomateo the seeds and staff not the hard part and work like magic hope it helps with your little one