I would contact Springfield leather and see if they could send you a sample of their milled veg tan. It is flexible and has a soft hand. It might be what you are after. I know they carry 3-4oz. They carry 2 thru 6oz.
I really like the block finish method of dying , as YinTx said it is not easy to do right. I have only tried a couple of times and not nearly as well as your project.
No need to be intimidated, your work is great. You can see the love and craftsmanship put into it. That will be with him for many years. Thanks for sharing.