It will be a hard decision to make, my first budge sheldon got a disease (which he probably carried with him from where he was bought from) he went downhill quickly and could not keep food down. I tried to syringe feed him and the vet said they could keep him on a drip for his remaining days but I made the choice to end his life peacefully there and then at the vets. Only you will know if its time. budgies hide illness very well so often when it becomes noticeable its often nearing the end. if he does not seem himself anymore and can get no quality of life only you will know whether to make that decision. sending hugs your way - I know how hard it is