Rat abscess or tumor

Question
Hello I really need your advice.  I have a male rat Jack who is about a year and half old.  He had an tumor that the vet said had abscessed last year.  We put him on baytril and the abscess popped also before we got him in for surgery him and his brother Joey had actually taken out what I had thought was the tumor.  He completley healed up his fur grew back and all was fine.  

Unfortunatly, it came back.  At first it was just a very small lump that did not bother him in anyway.  there was no scab just a lump underneath.  (by the way this is also on his neck) Sadly my fiance and I have been planning our wedding and were buying a house so things were very busy and my lil rattie had to be as he was for the time.  In the process of moving the lump has grown significantly almost overnight it doubled and then tripled in size and now has opened.  First we thought it was scabed due to the fact it is always resting on ground but now we see that a part of it has popped and there was puss inside.  He still climbs all around his 5 level cage, eats, drinks and plays.  In the past 3 days he has gotten very lathargic and doesn't do much he is still eating but has gotten so skinny for my once "fat" 1+ pound boy he now doesn't even look himself.

I love him so much and only want what is best for him.  I am so upset with myself and my fiance for letting it go this long. Do you know of any good certified exotic vets in our area (lone tree colorado)  that would be able to do surgery or is it too late for my precious baby?!  He truly is the most wonderful pet and I don't know how I could deal without his loving personality in my life.  Please let me know what you think and if there is anyone you could refer me to.  

Thank you!

Answer
Can you take a photo of this for me?

Also, here is what I have found as far as Vets go.  Although this vet is NOT a certified exotic vet, he is very skillful with small exotic companion   mammals and has a special interest treating them.       

Dr. Bill Guerrera
Facility: The Animal Doctor Exotic
Address:  1705 W. 10th Avenue
 Broomfield CO 80020
Phone:  303-466-8888
Website: theanimaldoctor.org