You have gotten many different answers here. Try to remember that everyone is different. My hubby likes to take his time. 2 hours in bed is not unusual. I used to be much more prone to a quicker cllmax, but he has conditioned me to hold off a bit. Now, a "quickie" isn;t even in my vocabulary.
The best way to figure this out is to be sensitive to him and choose an opportunity to ask him what he thinks the problem is. Be gentle and considerate when asking. He may be embarrassed at first so he may not come right out with the answer but if you arte sincere in wanting to solve the situation he will likely come around and tell you within a short time.