The earwax could simply be that the ears are having to deal with dead mites and the muck they made during their stay - but if it goes on for a long time then suspect a fungal infection.

You could try giving a probiotic - as that will help to sort out an out of balance gut even in a carnivore. At 5 weeks of age the kitten should have still be suckling and getting all sorts of beneficial stuff from the mother, so it could well be that his gut lacks the right sort of bacteria. Now he is older he should be eating mostly protein from meat or fish, though you could try scrambled egg to see if that helps with the runs.

Anne Croucher