Hi

What do you use to clean the area? You need to remove all scent as if he can smell where he's been previously, he'll have more inclination to go there again. He won't really understand that he's done wrong, cats aren't spiteful (usually!), he's understanding more the negative body language you're displaying when you see the accident (I'm sure we've all rolled our eyes and tutted when we've found a nice present on the carpet )

Give that area of the carpet a good thorough clean with a solution of biological washing powder and water-use nothing ammonia based. Then a squirt of surgical spirit to really get into the fibres to remove the last traces (do a test on your carpet first before you use spirit).

As mentioned another tray or even a change in litter may help-cats are very clean, and in multi cat homes its worth having several trays. Basic training like you would with a young kitten can help in trying to get him remembering his litter training too. In the area where he's going the most, you could put a small piece of tin foil there. Cats don't like the noise or feel of this and it can help with inappropriate toiletting. Just be careful to use a fairly small piece only in the chosen area, otherwise you could spook him into not going near the tray at all.

It would also be worth trying a Feliway diffuser in case the problem is more stress related. This mimics the happy pheramone their mum would release when nursing to relax her young and make them feel secure. You can get the diffuser (use like the air freshners that you put into electric sockets) or sprays.

Good Luck