Does it feel like the baby is underfeeding at our age? 4 months with a…

Does it feel like the baby is underfeeding at our age? 4 months with a tail. I'm on pure Gv, I think there's not enough milk, maybe. I don't understand. Recently wrote that my mother-in-law gave mashed potatoes puree because when we sit down he stares directly at the food, tongue shows that he wants too. He stretches for a spoon with his head. Now I have constant thoughts that my milk isn't enough for him. Because during feeding he eats, eats, and after some time starts to get nervous, angry, I check by squeezing the nipple, is there any milk, but nothing comes out. Does this mean we need to switch to mixed?