Skinny chick, how should I help him?

[Eden83_haaretz]

I know this in part is my fault, I have this young chick, when he was little his mom left him and all the others would bully him, so I decided to isolate him. Gave him chick feed and grains and would take him out once a day to roam free with supervision. Of course he hasn't grown up as well or fast as the others, and even though I see that his crop is almost always full (he eats well, is actually quite active) I just feel him around and he's too thin for my taste.
My husband is always telling me I should let him roam free with the others, he's a chicken and has to learn to interact with everyone (I'm just a worrisome chicken owner, when he was little and with his mom they broke his leg, then when she left him he went to her and she pecked a good part of his chest and made blood come out; I just worry a lot for him)
Should I just let him be? He sleeps in the same space as the others but in a cage (once or twice he's escaped and seems to be fine with all of them) but then when I'm with them if one of the big ones comes close he cries and runs to me. Is he just a cry baby because I've overprotected him?

Your recommendations would be highly appreciated. All my chickens roam free, they sleep in a coop and I open it up as the sun is rising, they all go in by themselves and I lock them up. We're always on the watch out for predators (hawks, eagles, cats, possums, dogs, cacomixtle) during the day, they do come by once in a while.

 

[Eggcessive]

How old is he? Do you have other roosters in the flock? If you don’t, you could consider letting him loose for supervised visits and eventually integrate him. But if you have another rooster, he may get killed. When my hen was ostracized from her flock, she stayed on one side of the poultry fence from the others all day long, and then I brought her into the coop at night to sleep. To help him gain weight, make sure that he doesn’t have coccidiosis. And try feeding him some higher protein foods, such as egg, tuna, or canned cat food in small amounts in addition to his chicken feed. Probiotics may be helpful as well.

 

[Eden83_haaretz]

How old is he? Do you have other roosters in the flock? If you don’t, you could consider letting him loose for supervised visits and eventually integrate him. But if you have another rooster, he may get killed. When my hen was ostracized from her flock, she stayed on one side of the poultry fence from the others all day long, and then I brought her into the coop at night to sleep. To help him gain weight, make sure that he doesn’t have coccidiosis. And try feeding him some higher protein foods, such as egg, tuna, or canned cat food in small amounts in addition to his chicken feed. Probiotics may be helpful as well.

He must be around three months, he's definitely small for his age, guess the chick feed and grains weren't enough to make him grow bigger :/. How can I check if he has coccidiosis? His deposits seem normal I think...
I do have roosters (his father and two sibblings which are older than him; I will sell the sibblings just as soon as they are older). The hens sometimes also peck chicks that are alone and chirp (when they get nervous).
I will definitely give him high protein foods, thanks for the feed back.