there could be a lot of reasons for this problem. The first is that he could have Erectile dysfunction. It can be treated very easily with drugs.
Another reason could be that he is nervous or concerned. Being self-conscious about ones sexual ability can cause a man to go limp during sex.
Another reason, yet tied to the last one, is stress. If he is working all sorts of hours at a stressful job then that could be another reason.
Just be patient and make sure to tell him that you love him no matter what. he might get testy at this little problem but assure him that all of these problems can be easily fixed.
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