Hi Laura - Welcome to JustAnswer! I am a licensed veterinarian and specialist and will do my best to assist you today!
You are likely seeing what we call a Scrotal Hematoma.
Okay is it serious? He isn't experiencing pain for what I see and he is acting normal.
No - it is not serious.
It is unfortunately from a little bleeding following the procedure into the scrotal sac.
This is typically "treated" with rest, and a cold compress at the site for a few days, typically resolving.
If you notice it is growing, or becomes painful - then seeing your vet would be important.
Yeah the vet that did the surgery said that but i was just worried because it seemed to appear quite quickly. Thank you for the advice, I shall continue the cold compresses and rest.
You are very welcome!