What should I feed my 5 month old?

My milk production (BF) is dropping and he's STARVING. I've tried every formula on the market - even the Nutramigen and soy, and they cause severe gastric distress. The GI specialist says he can see me in a month! What do I do in the meantime? I've already introduced solids to try to fill the gap, but it's not doing the trick. Baby can only eat so much - he wants MILK. He's casein-sensitive (can't have any cow's milk) and has AR. Tried supplements and pumping to get my milk production up. It worked 2 months ago, but isn't working this time. Has anyone had this problem? Anyone tried Goat's milk? Other solution?