Can you take a pic Jeff? It really will help us figure out how to advise you on your poor boy. The lump is an abscess...and its either collected after draining from his ear under his ear, or there's something else. Its definitely infection right? Smells horrific when you smell it, and the pus is yellowy? I have been through lots of abscesses before, but depending on how it looks, and where it is, I may or may not do anything.
Normally with an abscess we encourage it to open up with damp compresses, then flush it out with saline to clean it out and keep it clean until it heals but not knowing what exactly it looks like we cannot advise that at this point.
The polysporin shouldn't be used because the abscess is really just a pus/bacteria filled cavity that the rat has efficiently walled off from the rest of his body (normally) and polysporin will encourage it to heal, but that means the top will close over and bacteria will be trapped inside and the abscess will happen again and again.
Facial abscesses can be many things...infection bursting through the face from rotting molars, a bite wound that got infected is a normal cause for an abscess, ?ZG with its abscessed surface, infected Salivary glands (like my boy Twiggy), etc.
Dr Munn is likely going to open that cheek up to drain the pus, then try to discover where its coming from. IF something happens, you may get Dr. Hammond, but Julia is a very good vet and will explain everything to you.
Try to forget that its opening since its doing what happens naturally, opening up so the pus/bacteria can come out, but keep trying to feed your boy the ensure with a syringe...he needs food/liquids and Ensure can be both.