Thin young rooster

[Dewberry]

I have a 17-18 week old cemani cockerel who is much thinner than his brother. He seems healthy and he's not being bullied. His keel is prominent when I pick him up (he's a sweetheart) and feel of him. What would you recommend that might give him a boost? He's very tall and has huge legs and feet that are larger than his heavier brother's. They eat Nutrena feed and just switched from starter/grower to having access to feather fixer and layer feed. He eats well and has normal looking poop. All of the chickens free range during the day and I've noticed that the cemanis are big foragers. They're all been wormed recently. Treats are generally sunflower seeds, meal worms, scrambled eggs and watermelon.

I haven't raised a lot of roosters except for the Faverolles that are built like little tanks but I really think he's too thin.

 

[ChickenCanoe]

I would first recommend getting one of HIS fecal samples and having it read for parasites.
Secondly, don't feed cockerels/roosters layer feed. They don't build egg shells so their kidneys can't handle 4% calcium in their diet.
At that age, roosters usually do fine with about 15% or less protein but if you determine an underlying problem he may need a protein boost for muscle mass.
It's good you have a brother to compare him to. Don't try to compare one breed to another.