Conne, I once rescued a wee budgie, she was a tiny bird with a tumour in her stomach as big as a walnut. It was very clearly visible and so heavy that she was unable to fly.
I took her to the vet to be put to sleep. To my amazement he examined her, said it wasn't pressing on any of her internal organs, and asked if she was still eating (she was).
She couldn't fly but used to pull herself up the wire of the aviary to join the other birds on the perch. It was very upsetting to watch her and I told the vet this, but he decided, to my amazement, to leave her be. He told me that when she wasn't eating or making an effort to get to the perch, I should whip her into the surgery and he'd remove the tumour (a risky operation on such a small bird, but he felt it worth a try once she'd become too ill to interact with the other birds).
On the dreaded morning I got the carrying cage ready but didn't make the journey, as she passed away in my hands. She'd been pulling herself up the cage and eating well just the day before.
Never have I had a budgie with such a fighting spirit, and I rescued many in my early days, had well over 100 at one point.
I do hope you will be guided by your vet. He (and you, of course) wouldn't want Mr Harv to suffer.
Btw, have you got a local Morrisons? Campbell's Condensed Soup on offer at half price at our local. I've bought a dozen tins. Added a tin of celery to my home-made veg soup today, it was lovely.