Hello and welcome to hc, I'm sorry it's under these circumstances.
Without seeing a picture, going on the speed it has grown and the black I'd lean towards an abcess coming to head.
I would go to the vets. If it is an abcess, they may flush it or leave it to burst. Either way some antibiotics and pain relief would be beneficial.
Older hamsters do sleep more. You can offer cucumber to help with water intake.
If he is still generally well I wouldn't leave it. If it is a tumour then I agree an operation at his age wouldn't be advisable but if it is an abcess it needs seeing to.
Ive had several hamsters live past 3yrs so don't write him off yet
