It certainly would be beneficial for you to talk to him, maryjane. He's really young to be experiencing this - and it's often the first symptom of a blood pressure or heart problem.
I will say too, it's often a symptom of a man who is unable to function due to having other sexual preferences - whatever those may be - that preclude him from being fully aroused with what the two of you are doing at the time.
This is a really difficult subject to bring up - we all understand that, but I would think it needs to be addressed, and not while you're in the act itself, but sometime when you are out of the bedroom but relaxed and alone. He may actually have no idea there's a problem.
It could be mental or medical. It is hard to say without having tests done. If you two are close, I would suggest gently bringing the topic up or perhaps ask him if he is satisfied with your sexual relationship. It is a broad question and it gives him an opportunity to talk about his problem (if he chooses too). If you two do talk about it, make sure you support him and do not give him any slack for it. It could just come down to him being nervous or there might be a medical reason for it.
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