Why is he late? Answer that for yourself then you can decide if it is a reasonable action or not. I will assume he has a "good reason" to be late in paying rent. Either his pay schedules is such to where he gets paid after the date rent is due, or perhaps he has so many other bills that have to paid at the same time that he thinks you will not mind paying the rent when do and he hits you back later with rent. Have you talked with him about this? If not then you need too. If its because he is partying on and drinking or things along those lines, by all means slap on a late fee. Tell him it is part of the rent that was slapped on from the landlord since you can not pay the whole rent yourself. If I knew that he was always late, I would adjust my finances to cover the rent and then pocket his half when he does pay. Most likely is far easier to save a little from each paycheck through the month to pay the whole rent when due then wait until he pays you. OR.. a strange thought this, you can elect to hold back the rent he paid you last month for this coming month - that way you will still make the due date since you already paid the full amount. Also, if you can swing it I would strongly urge that you get ahead in the rent - Just in case he fails to meet rent at all.

If you cannot make him pay his rent on time how on earth are you going to enforce a 'late payment' fine? You need a new roommate.

Kick him out ... or charge him more money. Either way .. works for you. Why do people ask the most retardly obvious questions on here.