As was mentioned before.. it could be diabetes.
You need to have time together... hours.
Get him hard over and over and don't let him cum.
Then after 3 or 4 rounds of that... then try it. It should work... if not, then he needs to see a doctor. He will be embarassed... make sure he knows that you support him and that you will go with him in he wants you to. (he probably won't.. not at first)
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