welcome to yorkie talk, and i am so sorry for the reasons that brought you here. as rescuemama suggested, maybe there is an alternative medication that can be given??? sometimes it's just a matter of getting the right dose or combination of medications. i do not have any experience with this so sorry i do not have any advice for you other than if you have not seen a specialist with him, maybe that would be the next step? you could get his records and at least go to a consultation to see if you have any further options with your baby. you can use this link to find a board certified specialist in your area.
ACVIM
i know this is very difficult for you and the family and it is so hard seeing them get old and sick. i have an 11 year old girl with cushing's disease, hypothyroid, pancreas issues, and an enlarged liver. it took several months and so much adjusting/changing medications but she is doing really great now. these are different conditions, but maybe that can offer you some hope. give him lots of love and comfort as i'm sure that you're already doing. again, so sorry you're going through this. sending hugs.