Hi, this is Dr. Maggie - I am going to try to answer your question today.
I would avoid the temptation to try to lance the abscess yourself. This is something that many cat owners want to do when they suspect an abscess but you can introduce bacteria into the wound and can get injured. (Because your cat won't sit nicely for you to do this.)
Often, abscesses will open up on their own as time goes on, but since you are taking him to your vet tomorrow he or she may end up opening it to speed the healing process along.
Often these are caused by bite wounds, because cat mouths harbor some particularly nasty bacteria.
Putting topical antibiotics etc. won't really do much for an abscess and will just give your pet something to lick off, so I would avoid that as well.
You can certainly try to keep the area clean with warm water, but other than that I would leave it alone.
Your vet will likely prescribe some systemic antibiotics after you get him in tomorrow.
Any other questions I can help you with?
No. That will be it. Thank you. Just wanted to make sure there was nothing i could do for him.