It can be hard to fight a big crack like that. It takes about a year to grow out a hoof, from top to bottom, so any good your hoof supplement is doing will take a lot longer than a month to show any effect. Probably closer to a year! It will take nearly the same amount of time to get rid of that crack, if it were to magically quit growing today. That is because the hoof grows from the coronet band downwards.
It is probably fixable, but your farrier will need to get aggressive with the rasping to relieve pressure on the crack to keep it from spreading even more.
My mare has a bad quarter crack I fought for about 8 months. It looked great, like it was growing out and doing well. Then rainy season hit and boom, it cracked right open again. I've come to the conclusion she may have to live with it forever. Luckily it doesn't seem to cause any problems thus far.
Your guy's crack isn't all the way into the coronet band (yet anyway) so I think there is a good chance you can grow it out if the farrier gets really aggressive with relieving the wall pressure on the crack. But I would get it tended to ASAP, because if it goes up into the coronet band it will be harder to get rid of.
I don't think hoof supplements do harm, and they probably do some good, but the most important thing is to get him trimmed and rasped in such a way as to relieve the pressure from the crack so it will quit cracking. Otherwise all the hoof supplements in the world will not cure it.
Also, some cracks are more surface, and some go all the way through the hoof wall. No one can tell how bad it is from the photo. My mare's unfortunately goes all the way through the hoof wall, almost like it grows with a slight overlap. There are surgical fixes for really bad cracks, but I don't have the facilities to keep a hoof clean until it takes nearly a year to grow back out. They call those hoof resections. I don't think your guy is that bad, but don't let it get that bad if you can help it.