Can I ask a few questions first...

Is he spraying (leaving urine marks on vertical surfaces) or squatting to urinate (puddles on the floor)?
When did he start doing it?
How many litter trays do you have, what type of litter and where in the house are they?
What is the relationship between the two kittens like?
Has he been checked out for a urinary tract infection or FLUTD by your vet?
What have you tried already?

Sorry to answer a question with loads more, but there isn't a "one size fits all" solution to this. Jen xx

Only about 3 or 4 weeks ago
we have two cat litter trays one in the hallway downstairs and one upstairs because when they were kittens they if they didnt use the litter trsy thats where theu were likely to wee on the carpet.At the minure ive got that gritty grey litter previously i had the pellet sawdust type.He is leaving puddles.They get on really well together as they are brother and sister.We have to try to keep the,m out of the bathroom because they will wee in there we thought it was strange why they always wanted to get in there then i found out that the girl i got them from kept them in the bathroom because the steps were high and they couldnt escape from her bathroom.No i havent had him checked by the vet,

thanks lynn

OK. If he's doing puddles that is more likely to be because he has a urinary tract infection or he doesn't like the current toilet arrangements (spraying is more usually a stress response). Your vet can check a urine sample to rule out a bladder problem. The easiest way to get one if he can be persuaded to use a tray (shut in one room with one temporarily) is to use a non-absorbent litter which your vet will be able to provide.
If a physical problem is ruled out then we move on to the toilet arrangements.

The first thing that springs to mind is that you've changed the litter. Cats develop a substrate preference for the material they dig in before toileting. The rule with litter trays is at least one per cat and one extra, so three in your case, situated in low traffic areas of the house (depending how busy your household is the hall might not be ideal) Once you have established what type of litter he prefers then stick with that. You also need to make sure the trays are cleaned regularly, some cats won't use a soiled tray. If he has developed a preference for carpet you might even have to use a tray with a carpet square in until he uses it consistently then put a layer of a more suitable substrate on top to re-train his preferences.
It's really important how you clean up any accidents, or the cat will go back and use the same place again and again, each time forming a stronger association. Use a 10% solution of biological washing powder and water, then overspray with surgical spirit to remove fatty residues. Don't use any cleaning products containing bleach or ammonia as these have similar chemical compounds to urine and encourage the cat to urinate in that place.

Even if you feel stress or competition between the cats is not an issue it might be worth using a Feliway diffuser to increase the cats sense of contentment. Have a think about what outlets your 'teenager' has for his natural instincts as keeping indoor cats happy and stimulated is hard work. The devil makes work for idle paws!

HTH Jen xx