This is a similar situation to an issue that I had with a black dragon wyrmling. There is a way to force an alignment change and it would take a simple Geas spell, although I am unsure about the moral justification for it. Forcing someone to be someone they are not is just as evil as anything they are doing.
Anyway Nitsua60 explained this method or rather most of it and we did a little bit of digging and found the Bytopia method.
"There are three ways to forcibly change something's alignment on the books. But none of them really work out well for you....
Forced attunement. Arguably the least dangerous option, if you can somehow force the dragon to attune a magical item you might change its alignment. Perhaps a Geas? If your DM sees that working, and you have any of The Book of Vile Darkness, The Eye of Vecna, or The Hand of Vecna on you, the dragon's alignment will change. To Neutral Evil.
To the Planes. Send your dragon on vacation to the Bytopia for 4 days ("Pervasive Goodwill" optional rule, DMB pp.59-60), The Abyss ("Vile Transformation" optional rule, DMG p.63) or the Nine Hells ("Pervasive Evil" optional rule, DMG p.64) and its alignment might change to Neutral Good, to Neutral Evil, or to Lawful Evil. (But Dispel Magic or Remove Curse will get rid of the Bytopian effect.)
Enlist the Slaadi. Unfortunately (?) this won't work on your dragon--only on humanoids. But in the interests of completeness, here's the third way to forcibly change something's alignment: If you can get a Blue Slaad to bring your (humanoid) target to 0 HP it'll automatically become Chaotic Neutral. Of course that's a consequence of it becoming a Red Slaad."
Simply modify from dragon to "whatever you want"