You should definitely NOT try to make him feel guilty about this.
We each have sexual responses, fantasies, etc that are just a part of who we are whether we are born that way or if we are acculturated to it or some combination of those two factors. I'm drawn to redheads. God knows why but it is so. Yet all my relationships (I'm 70, so there have been a few) have been with partners either blonde, brunette or black-haired. Never once have I thought negatively about those mates because of their hair color.
And, it is entirely normal for you to have anxieties in the manner you've explained above. We all have them. Am I too short, too fat, too skinny? Is my nose ugly? It's a curse we all suffer from.
You note that he was embarrassed/ashamed to share his feelings about breasts and that he had never previously shared this personal information with anyone else. That he did share it with you is a very positive sign about both of you and your relationship.
So, any thing you might say or do that would make him feel guilty is going to hurt your relationship far more than enhance it. You ought to move in the opposite direction. Allow him to feel ok about this part of himself. Indeed, help him to feel ok about it. Be playful, not serious.