My son has rented an apartment, this month unable to pay rent on time----would like to know legal late fees in Louisiana.

Louisiana law, as I understand it, does not place a cap on non-payment late fees. The amount of the fees needs to be stated in the lease or orally agreed upon. "Excessive" fees can be contested in court, but it really depends on your definition of "excessive" when you compare it to the rent. Sounds like your son got a 5 day grace period, and then the fees started. I don't see them as excessive ($25/day isn't a huge sum).