i find the best source of slow released energy that wont cause him to be fizzy is oil, it can easily be added to his feed. it can be messy and some horses that are fussy might not like it, so you could add it using 'out shine' from baileys. i dont think this would cause him to put on weight if hes being worked 6 days a week. and make sure you remember the importance of forage, try giving him a slice of hay before feeding so the food gets digested slowly.

hope this helps, let us know how u get on xxxx