Hey moms! please tell me about formula and breastfeeding😬advice?

My baby is 1 month old,he was born 5 weeks early,iv practiced giving him formula and my breast milk since the 2nd week he came home,until now I think it caused gas problems..he poops now every 3 days but struggles,eventually it does come out..but is it normal? or is it because I'm mix feeding? I must say since iv given formula my husband would take over the night shift with 2-3 bottles which gets me the rest that I need to tackle the day! and being a happier mom! then I would breastfeed in the late morning and rest of the day, I don't want to stop formula but my husband says he regrets introducing it to him because of the gas pains, it dosnt make sense because fed is best! also he had an allergic reaction to gas drops,colic calm and gripe water...when Will this get better🙁 moms?