Please help

My baby is now 12 weeks old, the doctor finally prescribed me pepti 1 at 2 weeks old and it helped him so much in just the few first days of taking it. Then at 8 weeks he had his vaccines and had diarrhoea for two whole weeks, one day we even ended up in hospital with suspected dehydration. After another week of diarrhoea I have taken him back to the doctors and they have now given me a trial of alfamimo. He doesn’t seem to like drinking it and does spit it out but is still taking good feeds. But he seems to have gone backwards like when he was on cow and gate. Seems really agitated and still has diarrhoea. I just don’t know what to do to help him. Any advice would be welcome please