The "damage" was just normal wear and tear after 9 months.
They often moan about that and really expect every cheap saucepan to look unchanged after years of daily use. They don't really have much of a concept of wear and tear (unless specified in a contract), however along with the rental agreement there is sometimes a separate bit of paper listing the furniture fittings with a vague one word description of the condition. This means very little as there's nothing to say it's the same item (yes, Polish law is this useless).
If court's so expensive, what does a tenant realistically do if a landlord refuses to return a deposit or keeps some of it unfairly? Nothing?
Update, there are pictures of the apartment's original condition that were attached to the lease.
I just noticed your update. Pointless unless the photo was witnessed by a reliable person, signed, dated and
a second photo (also witnessed by a reliable person, signed, dated) was used in court to prove the present condition the moment your son left the flat
with no possiblilty whatsoever that the damage occurred afterwards.
After all - the landlord did give the deposit back and the time for grumbling would be before, not after. Anything could have happened in the time since while the flat is not in your son's care.
Ask for receipts for the rent so that you can give them to the tax office as part of your yearly tax return. Watch as landlord gets frightened.
In short, if the landlord actually did try to take any action (and there isn't the same small claims procedure that the UK has) they would be laughed out of court and end up out of pocket too. In Poland they often try this sort of thing on, but it is bluster. If they keep pestering, Harry's suggestion about tax will put a stop to any silliness once and for all.