Call your insurance or your clinic head or whoever and ask how long your session should be. Exactly how much are they paying him for?
Next time you see him, ask him how long sessions are. "How long am I supposed to be here? My insurance says it is at least 30 minutes. Does that sound right to you?" Smile and act relaxed when you say this.
If that doesn't get him to straighten up, get a new pdoc.
The very first time I saw my t, she was 20 minutes late. I was steaming because I know they have people booked on the hour. The first thing I said to her was "how long will I have today since you r late? She said "You will get 50 minutes which is the full therapeutic hour." I've had no trouble with her since.
Also, bring a short list of things you'd like to discuss. Let the doc know you have some issues you'd like to go over. Maybe he thinks you have nothing to say.
If you let them get away with scrimping on the time, they will do it.