His weight looks good to me. He could use some more muscle, but that will come as you work him more. I personally like my horses to be a bit on the lean side. They're all fat right now, but I use round bales, so not a lot I can do about it, lol. I'm working them more, which seems to be helping, but there are only so many hours in the day
Just like humans, it's healthier for animals to be lean as well. If you can feel his ribs when you gently prod his side, then he's good. If you can feel his ribs when you just lay a hand on his sides, then he needs a little weight.