After some research, it turns out that the HD progression for OD&D/OEC elves is utterly mysterious. There's no consensus anywhere on how to go about calculating it due to the weird way in which the elf alternates between fighting man and magic-user. Here's what I've come up with as a way of doing it and I'd be very interested in hearing some feedback.
At 1st level, the elf PC rolls hit points for both classes and takes the higher of the two totals. Using the OEC parameters, that means he rolls 1d8+1 (fighting man) and 1d6 (magic-user) and takes the higher roll. From then on, when he levels up in one or the other, he rolls the corresponding dice for that class. For example, let's say he's roughly dividing his XP evenly between the two classes (by alternating between fighting man and magic-user each gaming session). That would mean he'd hit level 2 as a fighter first, so he'd roll another d8, because 2nd level fighters have 2d8 HD. When, a few gaming sessions later, he reaches 2nd level as a magic-user, he simply adds 1 hp, because 2nd level magic-users have 1+1 HD.
Now, let's say he decides to focus on his magic-user aspect for a while and plays a number of gaming sessions in a row as a magic-user, thereby funneling all his XP into that class. So now he'll eventually become a 3rd level magic-user and thus roll another d6 for hit points. Staying with his magic-user class, he reaches 4th level and adds 1 hp. Now, however, he goes back to adventuring as a fighting man and eventually reaches 3rd level in that class, so he rolls d8 hit points upon doing so. And so on ...
I know that it sounds like he's piling on a lot of hit points, but remember that he's advancing in levels at roughly half the rate that the other PCs are, so (admittedly not having done the math) I think it should balance out in the long run. Also, he maxes out at 4th level as a fighting man, so from that point on he can only earn more hit points using the much weaker magic-user HD progression.
What say you?