My advice is to be direct and to tell him how you feel and that the age difference is something that you acknowledge but don't find a problem, in fact, as you point out, you prefer more mature men.
He may be concerned about how his family, friends and colleagues might look upon such a relationship.
My daughter, who is 22, married a man earlier this year who is... I think he's 41, or maybe 40. He was divorced, which concerned me initially, although it turned out he had been the 'wronged party' in his marriage breakdown. I understand that his friends were somewhat concerned when he and my daughter started seeing one another, in fact some of them really frowned on it, although they were eventually won round when the relationship endured (they were concerned for their friend, didn't want him to be involved in a fleeting relationship and for him to get hurt again).
Her husband's former wife and their grown up children said some pretty nasty things about the nature of their relationship, about the age difference, pretty sick stuff in fact.
Some people may say some horrible things about a couple when there's a big age difference. But all the matters, really is the two people concerned. And as in the experience of my daughter and her husband, even some of the objectors or people who initially frown on such a relationship can be won round, can come round to the idea.