Hi,
how can I force the player to drop the object he's holding?

Now I issue "+use, -use" commands, but if you know otherwise...

I suppose you could parent a phy explosion to the object, but that wouldn't really give the EFFECT of dropping. And I'm not so sure phys explosion has parenting. If the -use works for you, you should probably use it.

yep, it's not bad but fails when the player doesn't hold an object, since forcing the drop this way actually make him pick it up if it's near.

Have you considered actually just forcing the player to leave the box on a button or something like that? Portal is a puzzle game and it's usually best to solve stuff with puzzles instead of doing something weird like making the box simply get released.
I think some context into what you're trying to do will get you a better answer than asking how to do your specific implementation.

Nope, it's part of a new mechanic, but thanks anyway

You could use a point_clientcommand and trigger an output to -use at a certain spot (OnTrigger, point_clientcommand Command -use)

You can't input -use only, "+use, -use" works.
I'll post a different solution.. if I find one.