Worst thing he can say is, "no, I don't have time. I'm sorry", in which case you're back at square one. Best case, he says, "sure, I can squeeze you in." So you literally have nothing to lose by asking.

Are you afraid of rejection? You're not a burden by asking

My therapist had actually MADE an appointment outside her normal hours just so we could see each other. I asked. Granted it was a one time thing, but if you really need to see your therapist (i.e., emergency appt), he may be able to make room. He may even be willing to adjust his schedule to accomdate your needs. You never know until you ask.