Depending on how badly galled your horse is, he may need to see the vet to check for infection if they aren't healing. We have had good luck with Furazone ointment for treating galls.
To prevent them in the future, check the fit of your saddle and girth to make sure that they are not the problem. Some horses are prone to galls, especially a young horse that hasn't had a girth on him much. Horses can also be sensitive to the material of the girth...a sheepskin girth cover is often good prevention. Make sure that the girth and cover are also fee of any debris or matted hair/dried sweat that may irritate the skin.