a diet that is low in sugars and starch yet high in fibre and oil will give him plenty of slow-release energy that will promote weight gain without exaggerating his natural temprement and making him fizzy, etc.
Speedibeet, Fibrebeet and Dengie Alfa-Beet are all fabulous products which are high in water (helping to hydrate the horse) and fit the above statement perfectly! Alfalfa is also useful for weight gain and quality muscle tone as it is high in quality protein as well as calcium, minerals and fibre - In a search I founf Dengie H-Fi Good Do-er to be the highest fibre energy feed (It contains 40% fibre compared to the 16-20% found in most 'high fibre' cubes).Oil is also very useful as it contains 2 1/4 times more energy than found in most other eneergy feeds, yet this is released slowly.
I'd recommend:
250g (1 heaped coffee mug) = (dry weight) Sugarbeet
1kg (2 - 2 1/2 large round scoops) = Dengie Alfa-A Lite (can change to Hi-Fi Lite/Good Do-er as he gains weight and reduce down to 0.5-0.75kg as appropriate).
200g (just under 1 coffee mug) = Blue Chip Original/Lami-Light feed balancer
OR
225g = Baileys Lo-Cal feed balancer
... in each feed.
ad-lib, good quality forage (preferably soaked/steamed hay to reduce dust and mould spores and to reduce its nutritional value further).
good luck!
xx