Also, is he on a phosphate binder and how many how often? With snacks too? Is his PTH also high? My was astronomical and it affected my phosphorus levels. We started Sensipar and they've come down substantially.
The diet stuff is overwhelming at first glance, but this is a great site to get a crash course. It is does say for hemo, but I'm on PD as well and when I'm needing to get numbers under control, this is really solid advice. Plus they have a recipe database.
https://www.kidney.org/atoz/content/dietary_hemodialysis