Assuming that he has made a high "ask," you can counter with a "lowball" bid, by offering about half of what he asked. If he asks 100 and you bid 50-60, you might split the difference at 75-80.
This is "standard" bargaining advice. But before you take it, check the "blue book" value of a car of similar make, age, and condition online. It's possible that even 50 is too high a bid, and it's also possible that he's asking 100 for a car worth 90, in which case 75-80 would be a more reasonable bid than 50-60.
