From the photo he doesn't look too skinny, and that was before you said he had gained some lbs, so my bet is hes fine. Many vets prefere horses to be slightly on the skinny side than the fat side because slightly skinny is way less detrimental to health than chubby! Ignore what other people are saying about your horses weight. If you don't mind a horse with some shape and tone, they shouldn't!
So many horses are overweight nowerdays because generally owners don't know what a fit, healthily weighted horse looks like. I a survey some welfare trust did, when people were shown a picture of a pasture pumpkin, they'll say 'good weight', shown a top class eventer they'll say 'skinny'. Some slight pointy bits and rib is ok, no tone and all covered in fat is not!
We lose ourselves in the things we love, we find ourselves there too ~Kristen Martz