Talk about what's going on. It's a scary, awkward conversation, but it's important. Also, you've been together 6 months so you should be able to talk to each other about important things.
Ask him to show you how to get him off. I didn't know how hard you could squeeze a guy's penis until one took my hand and showed me. Any number of things could send him over the edge, or hold him back. Maybe dirty talk in his ear pushes him to climax, or maybe stuffed animals looking at him during sex make him too nervous to cum.
Everyone is different. My boyfriend loves blow jobs, but I've never made him cum that way. He still asks for them though.