I'm mostly posting because I'm curious as to what people will recommend. If you switch to a ration balancer beet pulp, alfalfa cubes, and/or copra/Coolstance can be added for extra calories. The Coolstance may not be available depending on where you live, and some people do not like feeding it so if you do decide to try it just make sure to do a little research on it. Others have had amazing results feeding it so it may be an option.
What concentrate/grain are you currently feeding him? Many feeds are designed to be high fat, low sugar/starch and shouldn't affect the horses temperament. I personally like and feed Purina Ultium and Triple Crown Senior, but after reviewing the ingredient list, the analysis, and seeing horses on these feeds, I also like LMF feeds (I wish they were sold in my area, *sigh*) Sentinel Low Starch, and Buckeye Cadence Ultra. These brands have others that are good, and there are other brands with good feeds (Seminole, Pennfield.) I just have not experienced them and so cannot give much of an opinion on them. All of the feeds I listed are low starch, high fat, and on the higher end for calories. You would have to check the NSC levels if that is a concern, I only know that TCS is something like 11 or 12%. I have not had any negative impacts on the horse's temperament from feeding these.
Cost wise, it would probably be more effective to feed the recommended amount of a high quality feed than it would be to try and meet your horses dietary requirements by adding large amounts of alfalfa cubes, beet pulp, or copra. I'm saying that based on you stating that your horse is more of a hard keeper and that he is kept in consistent work. What level are you eventing at? That will have an impact on what and how much you should be feeding him. I have evented as well and take lessons from a few eventing trainers, most eventers even at beginner novice and novice require quite a bit of calories, it demands a lot of fitness from a horse.
If your horse has an allergy to something, is sensitive to sugar, or you just prefer to go concentrate/grain free, I recommend high quality free choice hay (I recommend this for every horse, but unfortunately some stables cannot do or will not do free choice hay and getting high quality hay can be difficult these days) and then go from there in seeing what additional calories your horse will need. I hope I helped at least a little :) And I hope to see eventing pictures next summer!