It’s time for a real change. The fact that this has gone on so long and that you are in orbit around his insecurities isn’t healthy for either of you in the long-run. He sounds quite invested in his perspective. The way to go forward is to take care of your side of the relationship. This means letting him know how difficult it is for you to continue coping when he is so insecure.
I don’t think you’re the person who can help. He needs to deal with this on his own. If you try to help it puts you both in the position of being imprisoned by his insecurities. It is also intriguing that he is choosing a characteristic that isn’t going to change so that he can lament about it. This is something he needs to cope with on his own. You can be supporting — but you are not the one who can fix this.
Finally, according to Wikipedia the average height for a man worldwide is 5’7″. While he may not like how to he is — he is actually taller than average.