Hi Petra. You could also try either home cooking his food with limited ingredients, or buying freeze dried. To boost his immune system, use Probiotics, you can buy probiotics in pill form, or use other sources such as plain yogurt, which a great source, Greek yogurt, which is the next best, or Kefir which has the highest amount of probiotics, but is more expensive. Adding ACV to either the drinking water or his kibble will also boost his immune system. Coconut Oil has many health benefits as well, and I have just started adding Kelp to their kibble as well. Rinsing his fur with vinegar and warm water will also help with many skin issues, it will condition the skin, but it is a natural antiseptic and will kill many bugs, parasites and bacterias.