What is Best Supplement for my BF?

My boyfrnd, feels that he has lost his 'erection' to some xtent and the amout of ejaculation is also low. But, I don't find any difference, for the past 2yrs. He is perfectly alright & functioning well and I really enjoy being with him. I think, he must be comparing with the previous yrs b4 we met. I worried to see him dejected & losing his confidence. So, can anybody help, with a good supplementary, to get back his 'lost energy'. Also, it should not have any side effects, which may affect his currrent stage.. Plz, help.