He looks alot like mine did/does. Should be nothing to worry or lose sleep about if the leuc is not showing any odd behaviors. Fecal would not be a bad idea to give you peace of mind, but my honest opinion is that he looks fine.
I am feeding both size ff and they are both eating, but I only got them 4 days ago...maybe they weren't being fed as much before I got them? They should put on weight if they are being fed daily and eating, yes?