Formula and breastfeeding

Hi moms ..

So I’ve been breastfeeding my soon to be 4month old baby boy . I will have to be going back to work part time in a month . I’ll like him to take formula as well as me nursing. He will only be in daycare 14 hr a week . I been slowly trying to sneak in a bottle of formula but he spits it right out and gets extremely upset , I’m pretty sure he’s use to me breastfeeding and that is why he’s refusing a bottle . This is so stressful 😩😭 I might have to cut down breastfeeding completely at some point . My plan was to breastfeed until he’s 6months . But I don’t want to make it hard on him or myself.😢 any moms have any tips ? Or suggestions on how to get him into formula ??