He either has a bacterial infection, irritation, or injury in that eye.
If the eye is stuck shut with sticky stuff or gunk, use a warm wet washcloth and place a soft part of it gently over the eye to loosen then then gently pull the eyelids open.
Then, apply a little opthalmic ointment, available at your feed store or perhaps the vet, into the eye gently and carefully. Repeat for several days until the chick holds it eye open normally.
save the 15 bucks for terramycin at the feed store....I tried some neosporin on one of my girls and it turned out just as good. Next day her eyes looked much better and she was on her way to recovery. I kept her in the dog kennel until she got her bearings and her appetite back and put her back. I think one of the other girls pecked her by accident and this is the result with mine hen also.