There is a reason that the section on dealing damage from falling objects is in the DMG. It's a rule of thumb for GMs to use, not a hard and fast rule for players to abuse.
If you're letting players use the DMG (as it appears you also did with item creation), you're going to have problems. Rules in the PHB are assumed to be good as is. Rules in the DMG often need to be tempered with common sense.
Falling object damage has always been wonky. Consider that a mature blue dragon weighs somewhere in the neighborhood of 20,000 pounds, has trained in a special maneuver to deal damage by landing on and crushing its foes, and still only deals 2d8+13 damage with this attack, as compared to the 100d6 (or 20d6, depending on how you interpret that paragraph) it would deal if treated as a falling object.
There are several ways of dealing with this issue. The easiest is to have a conversation with the player where you explain that you were incorrect to let him use the DMG as he did, and you'll be interpreting things differently in the future.
The other option (for players with harder heads) is to consider that, if the world really works that way, others will use that tactic as well. Have a huge flying creature of appropriate CR show up (say, a mature adult white dragon). Have its first action be to fly over the character at issue, fold its wings and fall from 10' and land on top of him. Ask the character whether he would like to roll 100d6 for damage, or if everyone agrees that a new interpretation is necessary.
As far as alternative interpretations, there are a wide variety of ways to do it (I'd generally look at the dragon crush rules for inspiration; considering that requires you to be three size categories higher, that's going to make it worthless for the PC right there); however, pretty much any sane interpretation is going to end up making this a non-viable combat option.
While I consider that a feature, and not a bug, that will depend on how upset the player will be at this change (and, justifiably so, if you have let this go on long enough that he's building his character and item choices around this inane idea).