Hello,
It hurts a lot to read about the suffering you are encountering.
It is obvious that when dealing with monetary law – I cannot answer your question without hearing from both sides.
Fundamentally, the matter is of course dependent on the question of where the leak was coming from. If the leak was from his apartments – he of course is obligated to repair the items and compensate you for the damage caused to you. If the leak was not from his apartments – he is not responsible for the matter, the HOA is the one who is negligent.
From your words it seems that the one who is responsible is the HOA, for the sewage pipes are their responsibility. But as stated, I haven’t heard all sides.
I hope that you merit to have justice rest amongst you, thereby bringing peace.
All the best,
Rabbi Yuval Cherlow, Head of the Tzohar Ethics Center