Wouldn't it work in a sentence like "By this time next year, I will have had a son"? Because I don't see any problem with "I will have a son", it's a simple statement about my wife/girlfriend being pregnant and us knowing the baby's sex.
The past tense form of the verb is what I don’t get. There were no tips and notes for this skill, so I’m trying to understand this ‘on the fly’.
I understand this is the "Future Compound" skill or something like that. It's constructed with the future form of "być" depending on the person, and an appropriate (to the gender, only 3rd person!) past form of a verb / or an infinitive. That's the general rule, if you have some specific question/problem, just ask :)