Frozen EBM has a metallic taste which may not appeal to him. Same cos for HA and soy milk as it may taste bitter. You probably need to start with freshly pumped BM in bottles to get him used to drinking from bottles or straw cups etc first, instead of latching. Then replace BM with other milk sources, so he adjusts to one thing at a time. May need "cold turkey", to let him starve if he doesn't want to eat or drink, don't give in to latching.. when he's hungry, he would eat or drink eventually..
keen to know