Well, and I've missed discussion about this too, so I'm sure someone will come in and destroy me, but ah well. XD For one, Solas mentioned being too weak after waking up (Uthenera?) to activate it himself, and given that these orbs were each focused on one of the gods and their power, he apparently really needed that power. Why he needed it is up in the air, though.
He says a few times (though by the Maker, I can't seem to find the quotes) that whoever activated the orb should have died, how could Cory have survived, etc. What Solas tells us in the game usually comes with at least a grain of truth, so overall I think he gave Cory the orb, realizing that Cory was not only an ancient being of immense power, but that he was one of the few beings alive with any actual knowledge of how these orbs work anyway. He did so out of desperation, thinking (or hoping?) that, while Cory would be powerful enough to activate the orb, the ensuing blast would kill him and Solas would be free to swoop in and take the now-active orb for himself. That, to me, seems like a very Dread Wolf-ish move.
I mean, just looking at things like the tale of Fen'Harel and the slow arrow, or even just his banter with Iron Bull in which he sacrifices all of his pieces in their mentally kept chess game to achieve his victory, I think Solas is someone who sees certain sacrifices as necessary and will do what needs to be done to achieve an end goal. This is why he ends up joining the Inquisition, after all.
Do I think it was a good plan? Definitely not, but from his dialogue, he is apparently pretty familiar with bad decisions. But do I think it still makes a certain terrible kind of sense? Yeah. xD
If you're looking for his actual motivations, sadly we'll have to look to future content for solid answers on that. I hope. D: