or pasture mix is a less exciting alternative. apparently most horses find it a bit more heating than nuts, but not to the point where they're unsafe, which can happen with oats. one of our liveries wanted a bit more spark in her pony, and rather than try something inbetween first, she got some oats. it took 2 of us to bring the mare in from the field, and the girl was too scared to get on her. they can have more effect than you want!
also, cobs are often naturally laid-back, so feed will only have a limited effect. how fit is he? getting him fitter will help him to find work easier, so he should be more willing.