Since I made the assumption the horse does have problems with keeping up in weight, I would advise unlimited access to roughage. When this is not possible, give at least 12 kg roughage, and spread it over the day (and night). 

For a concentrate feed I would advise Pavo GutSecure, designed for the needs of horses with a sensitive/disruptive digestive tract that perform light work. A 600 kg BW horse will need 1,5 kg per day. Spread this 1,5 kg over at least 3 portions per day. 

To support the guts even more, and besides support  weight gain, I would advise to add some Pavo FibreBeet. It contains sufficient protein and energy to support healthy weight gain, but besides it also contains the prebiotic soluble fibre pectin. Pectin contains the prebiotic inulin, which is very healthy for the digestive tract. I would advice to gradually build up to 600 g per day. If after about 4 weeks the horse does not seem to have gained some weight, it can be gradually increased to 1 kg per day. 

When switching to a new diet, keep in mind that horses need about 7-10 days to adapt to this new ration. Always take your time by gradually giving a little less of your current feed, and a little more of the new one.