How did you find out? Did he tell you? If he mentioned it to you what exactly did he say?
In my own personal experience many woman are too sensitive about they way they look. They keep on comparing themselves to young models, etc. and feel unattractive by comparison.
I assume your husband married you because he loved you and not because you were the most attractive woman in the world.
There will always be other, more attractive woman out there. By the same token, I have no doubt that you are more attractive than a lot of other woman. The fact that your husband may find another woman physically attractive is normal. It does not mean he does not find you attractive or is no longer interested in being with you.