Since he's 8 months, he's still got some growing left to do. Feeding more wouldn't be a bad idea, but don't overdo it. About half a kilo or so would be good to offer. Also, you might want to try another meat. Another poultry, if available (e.g. duck, Cornish hen, quail, or turkey). You can also try lamb or beef, but they are rich and might cause stomach upset. So if you see diarrhea/soft stools, don't be surprised. He should adapt during the suggested two week period though. Make sure to include bone, or something like chicken or duck feet, to help keep his stool firm. Do be careful offering large bones as they could chip or break a tooth. Some owners don't offer beef bones at all. My dog eats oxtail (with bone) fine.
I think probiotics (healthy bacteria) are a good idea. Antibiotics, on the other hand, I would personally be leery of right now since they destroy/attack both good and bad bacteria in your dog's system. Antibiotics can also make you feel worse while you're taking them. Apple cider vinegar helps digestion and is good for the skin. If you can find some, one tablespoon a few times a week should help (some owners offer it daily). You can also apply it directly on his paws to help with the irritation.