Candybear; you didn't say whether it was every day? I would demand 2-1/2 days worth of lunch out each week (that would be "fair") and I would let him know he's been late back "too often" and that you have been covering for him when the boss calls but are not going to do that but once a week (or whatever schedule/deal he cares to make; perhaps you can trade so when you're on a long lunch he'll cover for you -- not that the boss is "dumb" -- he'll figure it out even if you cover every time that when he calls at X o'clock the guy is never able to come to the phone).

I'd just be joking/pleasant and say you get half the week (both people get 2 days and you get every other Wednesday or something). But if he is unreasonable, calls your "bluff," or doesn't keep to an agreement, I wouldn't tell him about the boss calling and your covering for him; I'd just next time(s) the boss calls say "I'm not sure where he is, he hasn't gotten back from lunch yet." Simple truth. If the coworker is a jerk, he's not going to change and there's eventually going to be a blowup anyway.