Need some advice...new

Hello everyone.  Let me start off by saying that it's not breast cancer.  I'm on this board due to my own benign breast tumor but this is the only place that I could think to come to.

My dad has been diagnosed with some type of carcinoma (most likely not breast).  The pathologist is working on seeing if he can find the primary in what was biopsied (not the primary site).  He's also going for a full body CT next week to find the primary.  I know  a lot, far too much, about cancer and how it works.  I know that next week he's going to get some very bad news.  I pretty  much knew last week when the MRI showed a mass near his sacrum that it was going to be bad but still had some hope.  I knew they were pretty sure it was cancer when my mom said, he only needs a bone scan.  I know what a bone scan is for, they do not.  Now I'm sure it's metastic and it's bad.  Where the mass is, isn't where a primary carcinoma would form.  I'm pretty sure he's stage 4, possibly lung since he was a past smoker.  He has no symptoms coming from the primary.  His only problem was a literal pain in the a$$.  It started after shoveling snow this winter and never went away.  His back x-ray showed nothing but the MRI is another story.

I came here because I really do not know where else to go.  Next week the bomb is going to drop and while I'm over the initial shock which came long before the pathology, my parents have yet to get to the tip of the iceburg.  I explained all I could without truly explaining what stage 4 means in terms of prognosis and treatment.  I'm scared to death for my mom.  I have no idea how she is going to handle this.  My dad will either get agitated or turn in.  I fear my mom is going to have a panic attack or get a heart attack.  I don't know what to do but I'm very anxious about what she will go through.  Little by little I start thinking about the ramifications but I'm numb.  Not sad.  Not angry (maybe a little).  Not paniced.  Basically speechless about it.  I don't even know what to do.  How do I explain this to my 5 year old.  It can't be hidden.  One second I feel fine and the next I feel like the room is spinning.  I don't want him to be in pain.  He's already in that from this thing growing near the sacrum. 

What do I do?  What do I do for my mom?  I can and can't believe this is happening.  It's probably lung cancer since he's an ex-smoker.  How can you become stage 4 when  you have a doctor checking your bloodwork every 3 months for diabetes?  By now, there should be something wonky with his blood that should have raised a flag.  If it's in his bones, he should have elevated calcium or ALK Phostphase.  I'm speechless.  I can't even put into words what I really want to say on here.  I know this board is only for breast cancers, but I just don't know where to turn right now.  I have no siblings.