Feeding my skinny older gelding?

i have a 20 year old arab gelding. who is somewhat skinny. he's not dangerously underweight, but you can feel his ribs somewhat through his winter coat. i have been feeding him approximately one pound of grain consisting of 1/2 lb. of cracked corn and 1/2 lb. mixture of oats and spelts on top he gets 1/2 cup of rice bran oil. he gets approximately 3/4 of a bale of hay total throughout the day along with 8 alfalfa hay cubes. he would be considered idle and is on a regular worming schedule. my question is after four weeks of doing this should i be seeing any type of weight gain? if not when should i expect to see some results? if you don't think this is going to work please provide other suggestions. thanks