its quite difficult  to find a feed that sparks them up without adding weight, i had that problem when i first bought my 10yr old gypsy cob. but what worked for me was to get him fitter so we were doing medium work then i moved him onto dodson and horrel's active mix. it really perked him up without noticeably adding weight. although you should always try and see if the fitter he gets the more forward he becomes? 

Hope i have helped :) x