Yeah, I'm in Yarra Valley, so know the season, know the feed!
So I assume he's getting the Zero in an attempt to provide nutrition? The more I have learned about nutrition, I've come to believe horses generally need nutritional supps throughout the year, not just over winter. Nutrition is one of those huge & convoluted subjects!
I use Zero myself, as a 'low cal' treat, and also as a carrier for nutritional supps. Unfortunately, it's not a 'ration balancer' and won't give him an awful lot of what he needs. If you want to go for Hygain, their 'Balanced' isn't a bad product. I too have fatties & I chose not to feed this though, because of the quantity, so calories needed to be fed of it. Mine are on solely bush grass ATM, and they're supped with KER Gold and extra magnesium. I highly recommend looking into supping extra Mg, for a number of reasons, not least that he's likely insulin resistant if he was/is obese.