He's still very young, he has got his wires crossed somewhere. He is still young enough to not always be able to control himself when he needs to go, and if he is upstairs or in another room, he may not get to his tray in time.
If there is only one tray, and you are working, he may object to using one that is already used. Try adding another tray.

Make sure you have used a proper pet odour eliminator product where ever he has been to completely remove scent traces.

Don't let him in the bedrooms until he's sorted and retrained.

Try keeping him in just one room for a few days, until there are no more accidents. He just needs the msg reinforced.

Do NOT punish him or do anything to intimidate him, he might become frightened of using the tray altogether.

There is a litter called Cat Attract that you can buy that is very good for re training. It's pricey, but you could mix a good scoop in with your usual litter to make it go further.

If you have changed litter, try changing back to what ever the breeder was using. Kittens are easily confused by change.

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