Well, I believe you are wrong about that.
With the contract, his assignments are controlled by the Jackets, who are bound by the CBA, and CBA doesn't allow them to assign him anywhere but back to his CHL team.
Without an NHL contract, what is to stop him? Any agreement with the CHL team would not be enforceable by courts, or IIHF, as it would be established when he was underaged. So, really, he should be able to go as he pleases if he doesn't sign an NHL contract.
However. He would have to compete for a roster spot and ice time with pretty good players, with many overseas leagues limiting the number of out-of-country players on team rosters. Dumais may be too good for the CHL, but is he capable of winning a roster spot, never mind a top-6 roster spot, from some non-teenage players? So, he's likely to end up in Allsvenskan, DEL, etc, and not NL, SHL, Liiga. All said and done he may be better off stayng in the CHL.