If you dont clean them, you may find the commercial guy might find another cleaner to do his house, and then do his commercial unit - where are you going to be left?
Get on and do the commercial units place at least, stick your prices up a bit for the other places, so at least if you do them, you get good money.
If anything is said about why your not charging the commercial guy that much, then say its a favour because you do other work for him.
Depending on what they do for aliving you could end up getting some more commercial work, for a slightly reduced home cleaning.