Thank you for this information.
It's good that the diarrhoea has stopped. In a young cat, diarrhoea is relatively common and can have many causes including parasites, infections, eating something they shouldn't have etc.
I would agree that worming is the sensible thing to do, but it's important it is done with an effective product such as Drontal, Stronghold or Panacur. There are wormers available from e.g. supermarkets that are not very effective and will not necessarily eliminate parasites. Beaphar contains Pyrante Embonate and Praziquantal so should be effective against tapeworms and roundowrms, though not hookworms or whipworms.
You say that when he sits he seems uncomfortable and is licking his rear. Is he walking strange or limping at all? If not, it may be that his rear end is a bit tender from the diarrhoea or if he has been licking. If you can, check to make sure the anus is not red and inflamed and there is not dry poo which may be caked on and irritating him. Bathing the area with cotton wool and warm water may help.
Another possibility is that his anal glands are full. This can certainly cause a bit of discomfort when sitting. A vet would have to check this for you and, if they are full, can squeeze them to unblock them.
His 3rd eyelids being on show is not unusual after a bout of diarrhoea and/or worms. This is called 'Haw's Syndrome' and it occurs when the gastrointestinal system has become inflamed. Even when things settle, it can actually take weeks or months for the eyelids to return to normal.
If you are sure the diarrhoea is gone, great! If not, we need to address this.