Cat care

Went to the vet first time for my cat who wasnt eating they said he has gastritis they gave him 5 day medication came back ate for a day then stopped and vomitted white foam and got diarrhea then again went to the vet they said he has parasite got a medication couple days was fine then stopped eating again and vomitted foam and got diarrhea what do u even do at this point?

Male cat 8 months old unneutered

Edit: this all srtarted when i got my female cat..could it be linked?..also she isnt spayed also only eats treats and also did vomit most of what he ate