Hi. How is more respect a bad thing :). Did he ever told you what are the reason(s) that attracted him to you in the first place?
Ho i see. What he probably mean is that the age difference may be too much and that in a near future, he may not be physically aroused by you, yet still like or even love you. On that point men and women (and many animals) are naturally opposite. If we take gorillas, the male prefer younger female as their ovula have more chance to yield high fecondity with less defect and there is a finite amount of them at birth that only degrade over the years. For the female, they often prefer the older males (known as grey back) because they show longevity genetics and the sperm contrary to the ovula is always made new each day and even if it can be a fecondity problem with the amount of sperm, it will be less subject to birth defect and disease. So, even if we are civilized, those evolutionary factor are still playing a factor when it come to pure sexual attraction.
If you have pictures of your mother still looking good in her old age, it might help but this is something he might still have on his mind. Another thing that you can use with him is an accord that when you get lot older you won't mind if he goes with younger women from time to time (a deal easy to do now but that oen may regret later).