I'm sorry to hear Tigger is having this issue. which must be very frustrating for you.
The first thing to ask is, is he neutered (castrated)? If not, male cats urine tends to get very foul smelling at this age so neutering him would be something we need to consider.
If he is neutered already, great.
Another consideration would be that there is an underlying medical issue that is causing the urine to be stronger, but also causing the indoor urinating. Cats can develop issues such as urinary stones and infections so, if not already done, it would be worht having him examined by a vet and bringing in a fresh urine sample to rule out one of these issues. If he does have a medical issue you may also notice other symptoms e.g. increased thirst, decreased appetite, lethargy, over-grooming. However, we don't always get additional symptoms.
We would also have to consider a common condition called' FLUTD' which is most often seen in indoor, male, over-weight cats (but can be seen in any cat). This information has some fab information about what it is and how to prevent it: https://icatcare.org/advice/feline-lower-urinary-tract-disease-flutd/
In the absence of a medical problem / FLUTD, then we would start to wonder if perhaps it is a behavioural issue. Some cats start to urinate outside of their litter tray when they feel stress or anxiety. Stress in cats can be brought on by e.g. living with other cats, having a litter tray that has not been cleaned, not having enough litter tray options (there should be 1.5 or 2 per cat, they should have a choice of kitty litter so he can choose his favourite and be placed in areas of the home with little foot traffic), a new baby or person in the home etc. So we want to ensure his home environment is a calm place for him.
If we think anxiety may be a contributing factor I would suggest a wall plug-in such as Feliway and/or some calming supplements for a few weeks to take the edge off.
As you mention the urine is strong, this usually indicated he is not drinking quite enough. Does he eat wet food? If not, consider it as it can help kidney and bladder health. Alternatively, you can add water to the dry food. Another option would be to get a water fountain for the home e.g.https://www.amazon.co.uk/Spardar-Drinking-Water-Fountain-Cylindrical-UK/dp/B07DRG8DX5/ref=asc_df_B07DRG8DX5/?tag=googshopuk-21&linkCode=df0&hvadid=218787967608&hvpos=1o3&hvnetw=g&hvrand=3087613834765144363&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9046078&hvtargid=pla-549954572050&psc=1
Some cats dislike tap water due to the added chemicals but will happily drink a bowl of rain water that has been collected from the outside.
When he does urinate outside of the tray, avoid punishing him as this may only increase stress levels. It's best to ignore him for a few minutes. Be sure to clean the area with an enzymatic cleaner to eliminate the odour. Do not use any cleaners containing e.g. ammonia, as this will smell like urine to Tigger and he will likely want to 're-mark' the area. If he continues to urinate in the same spot, consider blocking it off temporarily with furniture or a baby gate.
In summary, I would want to ensure there are no medical reasons for this new behaviour.
I would also want to ensure the environment is not causing the behaviour.
Finally, we want to ensure his water intake is sufficient.
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