Starting Cereal

My little one is 4 months old I have been exclusively breastfeeding him. I haven’t had to pump because I’m a stay at home parent. His doctor told me to go ahead and start him on cereal and 1st stage baby food. So Iv been trying to pump to get milk to add to cereal for him but I can barely get one ounce from pumping. I know I have milk he is gaining weight good and never fussing at me when I feed him he actually gets chocked when he feeds then gets sprayed in the face when he lets go. Any ideas to help get milk to start giving him cereal?