Could you try dropping the kibble altogether and just feed raw? If you continue to feed both it might be better not to mix them and instead feed one or the other at every meal, not a mix of both.
If Arthur wasn't scratching when he came to you and is being fed the same it seems unlikely that his food is the source of the itchiness. Other possibilities could include treats or chews, whatever he's been treated with for flea prevention, his vaccinations, the soap powder you wash his bedding in.
