We use pity as a noun or a verb but not an adjective. So is used before adjectives. You should not say:
*You are so boy.
This is ungrammatical because boy is a noun, not an adjective. The same is true of pity:
*It is so pity that you cannot join us now.
Since pity is not an adjective, this too is ungrammatical.
Instead of saying so boy, form an adjective from boy and say boyish. It would be correct to say:
You are so boyish.
In the same way, form an adjective from pity and say pitiful. It would be correct to say:
It is so pitiful that you cannot join us now.
In this answer, the * symbol marks an utterance as ungrammatical.