[QUOTE=KiraSophia;8202776]
I have my QH gelding, who previously foundered, [/QUOTE]
What was the cause of the founder, if known, or even suspected?
in a large dry lot with hay and a ration balancer to substitute grass (Empower Balance).
I’m assuming you don’t actually mean “the Balance is a substitute for grass”, but that the Balance is the one designed for a grass/grass hay diet 
How much hay?
He’s been on it for a long time with no issues until this spring, when he dropped a lot of weight. There are no health issues that we can find, he’s properly wormed and his teeth are good. He galavants around a lot more now that he’s in a bigger dry lot, so that might be part of it.
Can you lay out what he’s been dewormed with, and when, in the last 12 months? “Properly dewormed” means a lot of things to many people, some of which end up being “didn’t use anything effective for anything”
I planned to switch him over to Nutrena Safe Choice Special Care, which he was on in the past, but I don’t want to take away the grass-replacing benefits of the ration balancer, so I haven’t fully transitioned him yet.
Ok, so my assumption up top wasn’t right - I hadn’t read all the way yet. Ration balances don’t replace grass. At all. Not any more than any fortified feed does. There are 2 types of RBs: those aimed at horses eating an all/mostly legume diet (ie alfalfa), and those aimed at eating an all/mostly grass diet (whether fresh grass, or grass hay). It’s not even that, it’s really about how much protein is in the forage they are eating, so you MIGHT actually need the alfalfa-centric RB for grass hay that’s 18% protein. The difference is largely in how much protein and lysine is being supplied, as grasses tend to be on the lower side, while legumes are not
Right now he’s at half and half with a couple of extra flakes of hay and he’s gained a little back. What would you do at this point? Switch him over entirely?
My answer will entirely depend on his reason for foundering. You may have an IR horse who is the smaller population of those who lose weight and become “hard keepers”. If that’s the case, the SC is not appropriate for him IMHO.
I also would want to know how much hay he’s eating, and whether it’s tested low NSC, or it’s being soaked.
There are much better hard feed alternatives than SC SC for a metabolic horse.