Make sure that his worming program is up-to-date and that you are worming for the right type of worms at the right time. Get his teeth checked. Make sure that the hay is good quality hay. If you are not very experienced, my advice to you would be to go for a conditioning mix or cube and feed the recommended amounts. The manufacturers of horse feed go to a lot of trouble to make sure that their feeds are balanced, so try and avoid the temptation to put in a lot of additives that could unbalance the mixture.If you buy a lot of diferent feeds and mix them up,not only will you upset the balance but you may find that they last so long that they go out of date. Make sure he is warm enough, a lot of horses lose weight in the winter because they are using the energy for keeping themselves warm. Trial and error is really the only way to find a feeding regime that suits your horse, what works well for one may not for another, good luck.