I have so been where you are. Knowing something is wrong. Not doing something "in time" because I got either fed up with medical people or fed up with "bothering" my child over something that was a non-issue---supposedly. And then the big news hits.
It makes you human.
I want to offer you some words of advice which I hope you will take in the spirit I mean them....which is about avoiding more pain than necessary for you or your child.
If your son is still nursing...check out kellymom under nursing and hospitalization. There is a pdf file about being in the hospital. It's primarily about having a preemie-ish baby, but it will help you begin to understand the hospital.
Second, the hospital. If you aren't medically savy, start educating yourself to become so.
Third, the surgeons. Have faith in their training, but not in them per say. I'm saying, don't put them on a pedestal. They will fall fast and hard if something (G-d forbid) happens that is not to your liking. They are people. PEOPLE. They are trained to do a certain thing. Not everything they think will agree with you or help you. Not everything will disagree or hurt you.
Best advice ever was given to me by a surgeon. I was complaining about something that had "happened" to my son. He said, "You (the mom, mamaverdi) don't have to convince anyone of anything. You just tell them what they CAN and CANNOT do."
This advice has helped me and my sons so much in the hospital. When I have forgotten it, I have stumbled. When I treat my surgeons and doctors as people, even peers, colleagues, ones who have a focus in something I don't, then we sail, bumb, through my less scathed than usual.
Another thing, do you know if he will be able to eat afterwards? Is this impairing his motor skills...oral motor skills? If he will have an NG tube, I suggest starting to pump for him. (I see you have a new baby.) You can also get donor milk. I'm not sure where you are located, but the donor milk bank in Denver is 303-869-1888. I would prepare for him not being able to eat.
Also, downtime in the hospital....bring things for him and for you. I find brainless stuff is usually best. Start collecting books and knitting, whatnot. Phone cards, etc. Will you be in a children's hospital?
I wish you G-d speed and a complete and speedy recovery for your son.
It sounds like they are confident that it is removable and that a recovery can be made. Keep focusing on ways you can help him heal and get through.
Please PM me if there is any advice, help, etc that you need. I certainly don't know everything, but I am happy to offer help with whatever I know, and research what I don't.