Is rice bran powder enough?

I have an arabian in his twenties and he is underweight. Lately with the heat his tongue has been hanging out and he bites it when he eats. Wow he cant even eat soaked grain. I'm wondering if i mix some rice bran powder in a bucket of water if it will hold him over until the vet can come tomorrow or if it is emergent and the vet needs to come right now?