The first thing to do is give an oral liquid form of selenium. This should be given twice a day, after feeding and at bedtime. A bit less frequent will do as well if you are concerned the selenium may be too much for your kitten’s system. You can also try giving two drops of oil in his food every other day, but make sure it isn’t fish oil because tuna may have mercury toxins that could harm him. If he still has diarrhea, or vomiting continues then you might want to call your vet for advice about what else you can try next. It could just be that this kitty needs more calories than usual so feed him more often until he gets back on track with his normal size stool consistency again!