If he were mine I woudl go with a GOOD vitamin/mineral supplement or a ration balancer which they are grain free and low NSC and as much hay as he would eat.. I hate to see any horse on limited forage.. I feel they should have and they actually do NEED feed stuff in thier system at all times to keep the gut moving
Are you sure they are ten lbs ?? Or are you on the HUGE three string bales??
It is actually not recommeneded to feed rice bran for long periods of time but I forget why
A good ration balancer will cost you about 50 cents a day and provide a GOOD BASE nutrition to build on and it properly balanced ... with him being older as long as you don't flip the Ca/P ratio you should be fine...
A ration balancer will not make him hot and will help him in digesting his other food stuff...