yes, do keep trying. Maybe you will find a magic combination.
Sometimes for very picky eaters, a little bit of black strap molasses could be added to the block recipe to make it a bit more appealing.
Maybe, you could try adding some molasses to the balls? Not all balls, but try a couple and see how that goes?
Just use whatever minimally processed molasses you can find.
Add just a little bit to give it a flavour/taste kick, so to speak. Maybe, he will like it a little bit sweeter.
Maybe, after you make the balls, roll them in finely grounded nuts for an extra appeal.
Does he still eat his avocado?
Because in the meantime, you could try to measure a daily vit. serving (it is listed on the package with vitamins) and roll a piece of avocado in it.
But it will be best if a "solid" food solution could be found, like balls or "doctored" blocks, because with dusting it can be very hard to keep track of what actually gets eaten.
But avocados or anything sticky like that is better than dusting almonds: vit powder does not stick to almonds and gets brushed off.
Please share when you finally find the magic combination