Back to breast after bottle

My son is a month old and mostly breastfed. He gets two bottles of formula a day as recommended by his pediatrician. He still prefers the breast to a bottle but he gets frustrated that the breast takes more effort and is slower. He still takes it but the whole time he squeezes the boob and whines and kicks his feet like he’s desperate to get more out and it breaks my heart to see him so upset. He eventually gets satisfied and I get 5+ ounces every time I pump so I know there’s enough in there he just gets impatient. How can I fix this? I’d like to keep the formula in if at all possible but I don’t want him to be upset