What is the ruling on one who is given an opportunity to learn the Sacred Knowledge, but fears hypocrisy (riya’), while knowing that abandoning good deeds for the sake of people is hypocrisy, and fears that he will not know whether he was sincere in it or not? Is this a whisper (waswas)? And what should he do?

You are afflicted with waswasah (whispers/obsessive thoughts). The cure for it is to ignore it and pay no attention to it, and to strive in obedience to Allah while asking for sincerity and repelling ostentation. Seeking knowledge helps in getting rid of ostentation. These thoughts are from the plots of Satan to divert you from worship, so do not let him gain power over you and proceed on the path of obedience.