Suz - as the mom of 3 boys who are close in age - stop trying to be fair. Maybe - just maybe - easy child has your number? He knows you work so hard to be fair and so he is using it to his advantage.
Really - young men are their own species. My guess is that easy child has already forgotten about the conversation. Like every red blooded young man - he wants wheels and he was willing to play every card he could think of to get them!
And - in my opinion - girlfriends can aggravate the issue. They're practicing to be evil DILs someday 
In my own experience with my sons' girlfriend AND my own SILs (4!) - it may be the girlfriend who stirred it up a bit. My difficult child's girlfriend used that tactic a lot to the point where she actually had the gall to challenge me about granting permission to PC20 to do something (he asked for permission in her presence) that I did not allow difficult child to do at the same age. I was flabbergasted at her nerve! Now that she and difficult child have broken up - he has opened up about the pressure she put on him regarding his relationship with us. Of course, it's an extreme example - but it wouldn't surprise me if your easy child said something like "I wish my parents would just buy me a car" and she encouraged him by comparing his financial needs to his troubled older brother's financial needs.
Ok - I am projecting way too much here - but just food for thought. most importantly - I've learned that a lot of what my easy child's say and do "in the moment" is only reflective of them "just in that moment" and not indicative of long term or deep rooted feelings! (what you see/hear, is what you get!)
