Sentence: He spoke to me as a father to a son.

[Navi]

1-He spoke to me as a father to a son.
2-He spoke to me as a father does to a son.
3-He spoke to me as a father would to a son.

In which case:
a-He wasn't my father but spoke to me in the way a father would
and in which case:
b-He was my father and spoke to me in that capacity

[Haihao]

1 = a, (as if would be better); 3 = b, I think.

[Jamie (K)]

If you use the word "as", it nearly always means that he was your father, or at least it could be interpreted that way. If you want to indicate for sure that he wasn't your father, you have to use the word "like".

a - He spoke to me as a father to a son. (He's your father.)
b - He spoke to me like a father to a son. (He's not your father.)

Notice the difference:

c - I came to this country as a tourist. (= I was a tourist when I came to this country.)
d - I came to this country like a tourist. (= I wasn't a tourist, but I arrived the same way a tourist does.)