I don't think you'll be forgetting it. I would call him out on it now. Ask him if he wants to reconsider his response (this may give him a clue that you know). If he still lies then outright tell him you know about the comment on Instagram and ask what the big deal is in admitting it. If you are satisfied with his explanation, end of story. If not, mark as strike 1 and go on from there (giving another chance). I would only end it if response led me to believe there was more going on than meets the eye. I wouldn't be too eager to trust in a long distance relationship. Try not to give away your heart to him.