Still having issues figuring out what to feed my gelding...?

so i've had him on purina strategy since ive got him (3 months ago today) and it did what i wanted, which was get some weight on him. he's at almost ideal weight now. the thing is, the grain is making him hyper. i know because one morning i couldn't make it out to feed and that night i rode & he was better than ever. he wasn't nervous, he wasn't jumpy, he listened well, he wasn't go go go. i thought maaaayybe it was a coincidence, so i tried again and same thing. so now that he's at a good weight i'd like something to maintain his weight without grain. he's a 15 y/o gelding and on 24/7 pasture. he gets worked about 30 minutes a day either riding or round penning. in the winter he will get hay