It's also worth making EBM ice blocks. He'll take it slower which will give his tummy time to cope. Also get some probiotics into him. Inner health makes kid powder but as he's over 12 months you can break a capsule into cows milk and he can take that fine. Try to not give him larger amounts of anything. Even sucking on a wet flannel for hydration is better than a gulp of liquid as when it hits the stomach it spasms and with every vomit it causes more vomit. I would say if its this prolonged he probably has the rotavirus.