Hello moms. My child got gastroenteritis and had white coating on his...

Hello moms. My child got gastroenteritis and had white coating on his tongue, it was painful for him to suckle and we were admitted to the hospital. Antibiotics were injected, the tongue was treated. The son stopped breastfeeding since he got sick... We're home now but the breast still doesn't take. We switched to formula, but for three nights at night he cries in arms, sleeps and even formula doesn't drink. Yesterday he even threw up what he drank. Could it be that he is unwell because he stopped breastfeeding? Or is the formula not suitable for him? But I have been feeding him formula for over 10 days and I don't know why this is happening