Haven?t experienced the swelling, but at his six monthly check when Dylan was around 7 the vet picked up that one testicle was enlarged, and recommended castration as he thought it was a tumour. It turned out to be a benign tumour. The castration made no difference at all to him, he is just the same, hasn't gained weight
One of my previous spaniels had a swollen perianum, and that turned turned out to be aggressive cancer, had an operation but sadly died shortly after.
I personally would err on the side of prevention, as castration isn't a massive procedure compared to the other, but it's your choice of course. Perhaps some others here have some thoughts.
Lots of cuddles and love to your lad 💖🐾