Dark powers I have returned with a question.

I used the black carriage and had it trap one player (my 15 year old son, who just had to touch the butt) inside and take him off to the castle. The rest of the party is still escorting Irena to Valliki and are not chasing after the carriage. The way they see it they told him not to get in and he still did, so o no its the consequences of your actions. Session ended with them in sight of the gates of Valliki and him crossing the bridge into the castle.

So I'm going to do a solo session with him before the next group session to see if it resolves or he needs a replacment pc of some sort.

How should I do this?

1) Where should this happen in the castle? In the dungeon being tortured? Would it be a personal dinner? A walk through the castle?

2) What would Strahd offer him to turn on the party? If he refuses would Strahd release, kill, or imprison him? Would he turn him and send him back to the party?

3) what questions should I be asking that I haven't, and that should be considered?

   Extra info that may help

I know my son enough to know he's going to try and escape or explore the castle one way or another.

I think I can get him to turn on the party if I imply he won't make it out of here otherwise enough and/ or offer something good.

Also he is an half elf Oath of Vengeance Paladin who tattood his holy symbol on his hand. So if he uses it I fully plan on cutting off his hand if able to.