I agree, speak to him now, but try and keep calm. We sometimes expect others to know exactly how we think or feel without actually telling them, so he may genuinely not know what It means to you to keep your sobriety to yourself just now. He may also of found a way to check on you without actually letting on to your cousin. Is there a reason he might not trust you? If so it may take time for him to believe you, but the more you all to each other about your concerns and fears, the closer you can grow.