Why can't he use the turn undead ability of paladins?
Why can't he cast spell like other paladins?

Is this because he is an inquisitor? Or is it specific to him and if so why?

PS: I've already posted this today once and saw my message but couldn't find it after returning to the forum. I hope I won't spam but if so, it is not on purpose.

Yes your right it's because he's an inquisitor kit. He gets true sight and dispel magic as abilites but can't turn undead or cast cleric spells.

If you click on the Kit Information button, it should tell you that those are some of the disadvantages of being an Inquisitor. According to the manual, the other restrictions are no "lay on hands" and no "cure disease".

<font color=white> You can always look up the posts you made, just click on your name (in the list above the Thread Main Page, Like BG II - SoA)
Or left of a post you made (with your member status and NickName in it), press on your name and then go to: <font color=yellow>recent posts</font>

There is a list of your previous 50 posts... ;) so you can look up old threads of yours and/or posts you made in this forum. </font>