Hello. I am sorry to hear Buddy is having some trouble. My name is ***** ***** I would be happy to help.
It is not uncommon to see swelling or even whats called a Seroma develop after neutering if the dogs activity is not restricted and they are licking the area. A seroma is a collection of serum that is produced during the healing process.This develops due to inflammation. This fluid will be reabsorbed if we can get him to stop licking.
I am surprised that they did not send you home with an E collar. You need to get one tonight if possible. You can get these at most vets offices and pet stores. I know your vet is likely closed but you can call your local animal ER and they should be able to just sell you an E collar without seeing him. If you do not stop the licking he could not only cause infection but he could cause the sutures to open resulting in much bigger problems.
If your vet did not send home any pain meds while it will not reduce pain you could try giving Benadryl to help relax him. This can be dosed at 2mg per pound of body weight to make him a little sleepy.
If he will allow you to hold a cold compress on the area for a few minutes every 2-3 hours that will help reduce swelling.
The most important thing is to stop his licking if you don't this will just get worse.
Of course there is always a risk that the swelling is due to a bleed but that is very rare and this is most often caused by licking and activity as I explained above.
I hope this helps give you some direction and he feels better soon.
Please reply back if you have additional questions and I will be happy to continue. If you have no additional questions please take a few moments to provide a rating so that I am compensated by justanswer for assisting you today.