This is a habit that has been going on a long time. Get him vet checked in case UTI anyway.

I have a multicat household (3) and my neutered male was also weeing inappropriately by the front door. I too tried cleaning, multi litter trays, covered, uncovered etc.

In the end what i found worked was:

1. Getting a Feliway difuser which I plugged in the hall - location of frontdoor and main thoroughfare for rooms in house.

2. Got Cat Attract litter. This I used at first exclusively but I gradually mixed in OKO best litter (wood, clumping). So now it is a mix, probably about half in half.

3. I was lucky as he goes in 2 places only - near the front door in the mat well or near it or in the en suite bath - so both areas easy to wash down. But I removed front door mat, and covered with plastic bin liners at night topped with 2 old towels. This was so most of the wee would be absorbed by towels and prevented soaking in by bin liners.

Litter trays placed in mat well and in area just beyond.

If this happened and it did, chucked the bin liner and washed the towels with bio detergent.

It took about 8 weeks for him to stop weeing outside the litter trays and occasionally we still have an accident, but on the whole it has worked.

I still have the difuser and buy the Cat Attract litter.