I understand you might not want to talk about it with us, but I strongly recommend you talk about it with other parents who have walked this path already.  Your idea of taking out a second mortgage might not be the best plan of action if you're going to need to take any significant amount of unpaid time off work.  Other parents can also give you an idea of what things it's worth spending extra money on out of pocket and what things it isn't.

There are organisations whose sole purpose for existing is to help parents in your situation access services you probably don't even realise are out there.  The social worker at the medical facility where your son will be receiving treatment should be able to hook you up with all kinds of help, both practical and emotional.

If your employer is a large company, look into whether they allow donation of leave time by other employees.  Some companies allow this and it can be extremely useful for helping people maintain an income when they would otherwise have no choice but to take unpaid leave.  Talk to your company and see what employee welfare programmes they have which might be able to help you.