Do I capitalize s on "son" when it's used casually such as "Say hi to your pa for me, son."?

Some honorifics act as complete replacements for a name, as "Sir" or "Ma'am", or "Your Honor". I don't see why 'Son' or 'Dad' should not be afforded this honour. Essentially, only nitpickers are going to argue the toss if you choose the version that they don't like.

In the past, instead of a boy's name, men commenly called boys boy, kid or son, even sonny.

Imagine Humphrey Bogart saying in an old, black and white Hollywood movie:

Tell your pa I'm here, boy/kid/son/sonny.

But this is not a title used in place of a person's name and, therefore, should not be capitalized.

Compare

1. Give Dad my love.
2. Thanks so much for the present, Auntie!
3. I miss you, Son.

As noted by Edwin Asworth in his answer, when used in direct address, honorifics are usually capitalized. This applies when they are used alone or as part of a name.

Please help me, Doctor.

Please help me, Dr. Jones.

But as discussed on the website of the Chicago Manual of Style, capitalization of terms in direct address also goes beyond honorifics to encompass virtually any word used in direct address in lieu of a name.

I love you, Father.

Oh, Cutie, look this way!

Hey, Buddy, can you spare a dime?

Excuse me Son, but quit while you're ahead.*

*(Said to me by a judge when I interrupted him. I could hear the capitalization in his tone of voice. I shut up.)

One of the grayest areas ever in grammar. My take.

1. A mother always calls her son Hon. Short for honey, of course. It get's a cap. If she uses hon/honey/dear/dearest/luvvy on occasion, no cap. Is it a constant replacement for actual name? Cap it. Used on occasion? No cap.

2. His father always calls him Son. Gets capped.

3. His grandfather always calls him Kiddo. Cap it.

4. The boy always calls his grandfather Gramps and refers to him using it. Cap it. Same with Gran or Granny. All the time, as if it were a name, cap it. Sometimes, no cap.

5. His newspaper route boss calls him kiddo every day. No cap. He's not using it as a name substitute, just as a nickname. He's calling him a kiddo. He calls other boys kiddo as they pick up their papers.

6. The guy at the store calls him sonny every day. Other kids too. No cap. He's not using it as a name substitute, but as a common nickname. He calling him a sonny boy. Do they have a relationship? Is the boy there all the time, do they talk, interact? Does he call other boys buying candy/gum whatever something else? Maybe cap it then.

7. His best friend always calls him Doofus and he always calls his friend Dorkus. Cap it. If they use different nicknames at different times or use their actual names too, no caps.