I'm not convinced it ruined him. On TNG, Alexander embraced "human" values and pretty much rejected all things Klingon so of course when he gets older and makes a foolhardy decision to join a Klingon crew in the middle of a war to get daddy's attention, he's going to be out of place and bubbling. There's nothing in TNG that would lead me to think that Alexander's appearance in DS9 is out of synch with the character other than the obvious bump in his age to make him older. He really should have still been in the 10-13 year old range, but i guess with trek you can just say that he appears older because that's how Klingons age.
I think you've nailed it. Alexander must know that he's not really fit to serve on a Klingon ship, but not only is he hoping to look better in his father's eyes by embracing his Klingon heritage; in Starfleet he'd be in his father's shadow, and probably run the risk of constantly being compared to his father. Worf has his flaws but he's an excellent officer, and Alexander may fear that he can't fill those size 18 boots of his father's.