If you can smell other cat's wee round the catflap so can Charlie. Try washing it off regularly (if you don't already). Use a solution of biological detergent as that will kill the smell.
Not sure how you are wiping his urine outside, but it would be effective to put some of the soiled litter from his tray onto the earth in the garden as that will maintain his scent profile.
Not sure how you are wiping his urine outside, but it would be effective to put some of the soiled litter from his tray onto the earth in the garden as that will maintain his scent profile.




Mind you, the neighbours might escalate as well.