Buying and choosing feed can be really tricky because you don’t want to pick anything that would make them too hot or give them too much energy if you can’t properly shed that extra energy.
The feed that a family friend helped me pick for my mustang was by the brand Harmony. Its a simple feed but has lots of nutrients that are really good for her without making her too hot and giving her excessive energy.
If the horse has any special dietary needs you can talk to your vet or someone who has or had horses with that dietary need or struggle because they might have some really helpful suggestions.
There’s also the discussion of not feeding them grain at all for the time being if a vet thinks that a really basic hay diet would work best for your horse for a little bit. But observing your horses behaviour and what you are doing with them along with if they might have a more sensitive stomach all come into play with choosing feed.