Around how long should I wait to see if he is gaining weight?

Seeing weight loss is always easier than noticing weight gain. You should generally see results within a week. What I like to do is take a belly tape measurement every couple of days and take a picture every week. I record results and compare pictures. Sometimes it can be difficult to eyeball weight gain in person because you see the animal everyday and won't notice but with pictures I can step back and take an honest look at the pictures and see results....or if there aren't any. Use your own judgement and if you can't see any results after a few weeks let someone else take a look at your pics. Remember every horse is different and gains/loses weight at a different rate. Also, weight gain will slow as you approach the ideal weight for your animal.