Constipated poo


My son is 4.5months old. We just started integrating formula and switching from breast milk. He currently gets 1/2 and 1/2. Poor baby now is spitting up and has been constipated. We gave him prunes to help with the constipation. However is his poo supposed to be this thick on formula? He had perfect seedy poo before when he was just breastfed, but after seeing this I am worried. Is this normal or should I be calling his pediatrician? Sorry for the icky picture.