How can I get my boyfriend to stop repressing his anger and tell me how he feels?

Talk. Tell him how you feel, and focus on your feelings, not on his actions, or he may get defensive or just shut down. Maybe you feel confused, unappreciated, or abandoned when he doesn't give you feedback on certain things and you want to feel more connected. Tell him you want to take the relationship to a higher level and you want to really know more about him, including his feelings.
Repressing Anger? Perhaps u can convince him to see a mental health professional for evaluation to find out what is going on , the other options would be to see a therapist for family therapy.