My boy had white line disease (or seedy toe as i know it) and my barefoot trimmer had me use copper sulfate mixed in with vasaline (to make it like a thick paste) and you pack it in. With areas on the hoof where it is quite deep, you can give it a good clean (my trimmer did it, not me, but she did where she cleaned it as well as she could and cleaned it with something (can't remember the name sorry) and then she would squirt some of the copper sulfate and vasaline mixture up there and then pack it in with cotton wool.) Maybe ask your barefoot trimmer what she may suggest and if she knows of this method. Cause they should know what to do.
Cause it is packed in, the copper sulfate lasts a lot longer in there. It's worked wonders with my boy and his is almost all gone. It helps promote the growth back too, and kills the bacteria of seedy toe. :)
For parts where the seedy toe isn't too deep and you don't need to 'pack-it-in', if you can do it everyday it would be good, but otherwise it's ok to do it every second day, just give it a good clean (a nail like what a farrier uses to put shoes on is good to use to get all the dirt out) and then just squirt the copper sulfate up in there and just smooth it over with your fingers.
Hope that has helped and hope you can get his seedy toe/white line disease treated as soon as you can. :)