Don't let him walk all over you!
+1!
I can't really grok your gaming group. Unless there are multiple groups you're involved in. In one thread, you're really concerned with pricing magic items by RAW. But, here you're asking about something that, to my knowledge, there is absolutely no D&D rules basis for. Barring a few niche uses like Trophy Collector, which is more about a game bonus than wealth.
I don't mean this as an insult in any way. I think some level of consistency, even just in the sense of the amount of willingness you're willing to wing it as a DM, would make your life easier. If they know you're going to make off the cuff rulings on such things, then everyone knows that, and won't worry too much about finding particular rules basis for it.
There's no reason, RAW, and arguably by common sense that he'd be able to scavenge a whole lot from an iron golem. It's (a) just a hunk of iron, not the most valuable material in D&D's game worlds, and (b) presumably beaten to hell. That being said, it depends how much of a plot point you want to emphasize with this. I could see it being very fun to hunt down and harvest various components so that magic items can be made. It could be nice character flair, or it could be a total pain, depending.
Alternately, you are well within your DM rights to fold this scavenging into the wealth rewards for a monster. Rather than getting a random cloak with griffin feather trim (or whatever is on those silly charts) you can say that he scavenges some rune-scribed iron that was not damaged by the battle worth the same amount of gp to an interested arcanist or collector. Easy.
As a side note, the argument that b/c you can do it in an MMO you should be able to do it in a table-top game is bullshit. We don't typically sit around in tabletop sessions for 4 hours while people grind through smithing 600 iron daggers, nor do we waste each other's time with having people walk for minutes or hours to get to the next plot point. We just fast forward. They are radically different mediums. And, it's not like you're saying he can't scavenge things, the question is what benefit to get from it.