Hello,
It can be due to erectile dysfunction which is psychologically induced. Talk to your partner openly about any fears or apprehensions, eat nutritious healthy food, exercise regularly and cut down on smoking or alcohol if you are taking it. Also ask your partner to indulge in plenty of foreplay or use a water based lubricant.
I hope it helps. It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.
Its a great problem for those who have a strong desire of sex and habitual of performing it everyday. but if u got a patience it is not a big worrry cuz the issue you have posted which your boyfriend is suffering can be cured. its of dysfunction problem he is suffering from and its due to some nervous disorder . and I suggest you to consult some good doctor nearby. its such a matter usually boys hesitate to share and feel shy to consult a doctor. i u can convence him and take to a doctor ..