I understand your problem. This is temporary problem and psychological in nature. Let me explain this. An occasional episode of erectile failure due to functional or situational reasons like fatigue, tension or own pressure to perform rather to enjoy is very common. This happens to most of the men especially at your age. Once this happen, you become apprehensive prior to next sexual encounter and have obvious concern of quality of erection during next act. You focus on the performance of your penis and try your best to get hard rock erection by heart. There is remarkable anxiety and pressure in mind. The problem starts here. The mind now is under pressure to perform the best rather than just enjoying the kindle . This DIVERTS the mind from pleasure to pressure. For constant erection, continuous exciting/romantic/erotic feelings should be there in mind. If the mind is diverted to concern about erection, one will lose erection. So, for now onward, he should simply enjoy the kindling of sex, feel the touch. Caress you and focus on the pleasurable sensations you get by caressing. Do not bother about erection or quality of performance at all. Just enjoy, enjoy and enjoy. You practice this for next 30 days at each and every act. This will definitely fix the problem. Just relax and go ahead. No medication required. I recommend psychological counseling with psychiatrist. Wish you good health.