Usually if you Court Chaplain is catholic, there's a good chance of an event firing offering you conversion.
If they are a member of your court (and possibly realm)you should see an option on that character's diplomacy screen where you can demand they convert to your religion. Make all your councillors catholic, as I believe there's also an event where they ask you to convert. If your siblings are heretics they try to convert you so this might happen with orthodox people as well, although I've never seen it myself. So i'll have to wait some 40 years before i get this problem solved. Problem is its not my Heir, its my main charkter and he is realy young king, like 22 with no kids yet. It usually doesn't take long to have your heir changed into his tutors culture/religion.
If you don't have any catholics just invite catholic claimants to your court. If he is too old (you need to start at age 6 to be 100% sure) just wait for next heir.