Thank you for this additional information.
It is vital we ensure he continues to be able to pass urine; an inability to do so is a true emergency and can result in a burst bladder or acute kidney failure.
If he can produce urine but is straining in the tray, he may well have cystitis. This is an inflammation of the bladder that results in cats feeling the urge to urinate even when they don't need to. We may also notice other signs such as mild lethargy or grooming of the area (as you've spotted).
Oftentimes, cystitis is caused by stress due to e.g. a change in environment or routine.
For a sensitive cat, something as simple as putting up a tree could do it.
Cats most likely to suffer with cystitis are those who are over-weight, eat dry kibble, live indoors and are prone to stress. We find those who live with other cats are most often affected.
Other considerations would include a urinary tract infection or bladder stones.
A vet visit is needed both to examine your kitty and also to analyse their urine.
It is quite likely he needs some anti inflammatories, to help settle the inflammation wiithin the bladder.