Barley, alfalfa, linseed jelly, peas/beans, sugar beet (speedi beet and kwiki beet are the same, just preped differently) and bran are the best for putting weight on, and ad lib hay
At first, I would buy the straights and mix the meals your self, then change to a complete feed if convenience is needed.
How tall is he and how much does he weigh? (15.2hh should = about 500kg, 16.2hh = 600kg) assuming he weighs (or should weigh) 600kg I would feed:
-about 10kg of roughage a day (just under 2% of bodyweight)
Then in 2-3 meals:
-1.5kg soaked sugar beet (300-500g dry depending on brand)
-1kg alfalfa chop
-500g oats (if you want)
-a few oz linseed jelly (if you can be bothered to make it!)
-a probiotic suplement to improve gut flora and reduce chances of ulcers in the stomach and hind gut.
-bran if he's fussy, or if he gets cold- warms em up good n propper (oat brans are harder to get hold of, but better if the horse has hind gut ulcers)
Although its hard to avoid starchy feeds when trying to put on condition, I really reccomend trying to get the starch intake to less than 20% as soon as you can. Speedi-beet has no starch, alfalfa very little, but unfortunatly grains have around 40% starch. It could be worth finding a lower stach pelleted feed (or a 'balancer' if $$ allow) instead of the grains if you suspect your horse might have ulcers or any other digestive problems. Check out the lables in store, or email companies directly for full nutritional analysis and content.