When I feed my son even when he's very hungry, he eats two to four tab…

When I feed my son even when he's very hungry, he eats two to four tablespoons normally, then spits it out. And he asks for more, opens his mouth and spits it out again. Whether it's porridge, puree, meat, etc. Besides fruits and baby yogurt. We barely gain weight, we might not gain any in a month. The allergist says feed him, but how can you feed him if he spits it out? ? I tried to soothe and sweeten it.