Not possible. I don't believe you should try. This is an intent I think no magick should be used for. It's sound's like a secret. Just keep it that way. He does'nt want you to know, so keep it that way.
You can not force your will on someone nor should try. You should talk to him first, as that is how it really works. If you want, you can always cast a spell on yourself that encourages people to tell the truth around you, however I don't know if you really want to do that. You can also cast a spell to better see the truth in things, either in a person and their intent, or in a situation your involved in.
Again, these are spells on yourself and not others, you shouldn't do spells on or for someone else unless you have their permission.
This is not a good idea. If it backfires, you could breakup.
Here are the risks:
?You become obsessed with him
?He leaves you
?Bad luck, long term
Plus, it just isn't that moral. If you don't believe your boyfriend is telling the truth, talk to him. Remember, spells are powerful, but if you communicate, you can fix the problem with no risks at all.
Maybe he wants to cast a truth spell on you? How would you react to that. If you found out he casted a truth spell on you, you'd probably break up, because it shows that you don't trust each other. Just talk it out.