Even if Louis XVII's imprisonment doesn't turn out as bad as it did according to the legend, I'm pretty sure he would still be traumatized by the events. After all, he was there when the crowd invaded Versailles then the Tuileries and his parents were sent to the Guillotine by the Revolutionnaries. Possibly, he also was forced to accuse his mother of incest unless that was butterflied away. And then, we have to account for his imprisonmenl: I'm not sure a kid would took well being imprisonned.
You mentionned his sister Marie-Therese: I think one could explain the fact she became a reactionnary because of what she went through. It's quite likely Louis XVII could go the same path she did.
That being said, a lot will depend on how he is treated during the events.
I agree that it will depend on how traumatized he is. I can't see the revolutionaries passing up a chance to ruin Marie Antoinette's reputation even more. That being said from what I've read on Wikipedia it seems that we can dismiss his supposed mistreatment by the Simons as fiction by later Royalist writers. I mean he wasn't educated as a Prince by any means but the very idea that the Revolutionaries had him raped by Prostitutes to try to infect him with venereal diseases has got to be pure fiction. My idea for the POD is Louis isn't mistreated after the Simon's departure or perhaps they never leave. As I said earlier the whole of Louis XVII's imprisonment from the time he was separated from his mother is hard to document. I'm inclined to dismiss the worst offenses as Royalist propaganda but the part about him being locked in a room alone for six months has yet to be dis proven.
As for Marie-Therese, no doubt that her treatment by the Revolutionaries colored her views later in life. That being said I wouldn't call her a reactionary. To me she was very conservative but she did oppose several of her father-in-law Charles X's actions, including the Four ordinances of Saint-Cloud. I think that it really depends on how he was treated and how much of the revolutionary education sank in. I think he'll definitely be conservative but how conservative is up for debate.
If he's treated fairly well, as he seemed to be by the Simons, then he might just end up more sympathetic to the Revolutionaries, or at least more like Louis XVIII then Charles X. If he's treated bad or similar to the legends, then I'll bet he'll be an earlier, younger Charles X.
Any guesses as to who we would marry? DO you think there might be a chance of him marrying Napoleon's OTL second wife, Marie Louise? Or would he remain single until the Restoration, like his cousin the Duc de Berry?