What is the religious ruling on refraining from asking about what is ambiguous to a person: Does it expel him from the faith or not?

The questioner is afflicted with waswasah (whispers/obsessive thoughts) concerning disbelief. He must stop indulging in these whispers and turn away from them. A Muslim is not held accountable for forgetfulness according to Islamic law, so forgetting the questions carries no sin. And abandoning the question is not disbelief at all.