Like Gettaway king stated it looks like some light nutrient burn. Considering this is the "runt" it may just need a little less food than the bigger siblings. Not sure how you're feeding, but assuming it's not soil amendments, taper down the feeding by 25% or so for this guy and see how they look going forward. Once you see the beginning of a deficiency I'm the bottom leaves, then you can up the feed to regular levels.
Best of luck!