Hi peppe,
things like this are often a matter of trial and error unfortunately, but i thought id just leave a note about his food.
i know you said he's been on the same food for some time, but, depending on the type of food, some companies tend to vary ingredients a bit and this can trigger things like itchiness.
a lot of dog food manufacturers are deliberately vague with the labelling of their foods, they put something like 'cereals' or 'meat and animal derivatives', rather than actually saying 'soya' or 'wheat' or 'chicken' for example. the difficulty with this is that while you can be feeding your dog the same food for some time, with the dog appearing perfectly healthy, if the company alters the ingredients slightly (mainly because they've found a cheaper alternative) this can trigger an intolerance in your dog. wheat and soy intolerance is quite common, some are even allergic to poultry. its therefore worth considering feeding a hypo-allergenic type of food like burns, where you have a full idea of the ingredients and there are no chemical additives.
i think you said you started adding yumega to his food, which is all natural etc, but he still may have an intolerance to one of the ingredients. so it may be worth seeing if stopping the yumega makes any difference first.
hope this is of some help. good luck!