Have you tried a "light" food? Evidently, the light food is much higher in fiber. My dog had soft stool issues and once I put her on Science Diet light it really helped.
Note: they offer both regular size kibble and small bites. Pick which ever you think he would prefer.
p.s. As another poster said below, try frozen green beans. That also helped my dogs at times when they needed fiber. Since they're frozen they're crunchy and don't have much flavor. Your dog might be more likely to eat the frozen ones. It's certainly a cheap option to try.
Maybe I don't understand BalanceIt the way I think I do. I view it as basically a vitamin mix and if I they have too much of a particular thing they just pee it out. Is that not the case? Also, now that you know what I'm doing is there something you could recommend instead of tallow? I want my dogs to be healthy but I'm also not a helicopter parent that thinks they can't ever have dog-McDonald's so I try and make dinner time delicious and fun for them but also healthier than store bought kibble, although I do think Hill's Science Diet Dry Dog Food is good food for the price.