He lost his way explaining his theory is acceptable. I have a feeling I'd omit the in, but inserting it would still be acceptable. Also, maybe something like this:What other expressions would you use if someone has more and more difficulty ( or discrepancy) explaining his own theory?
He lost his way describing his theory. (Anyway, 'he lost his way IN describing his theory'?)
He got all tangled up explaining his theory.
He got hopelessly lost explaining his theory.
He stepped all over his own toes explaining his theory.
Hope this helps.