everlast

Am wondering if anyone is able to offer any help/suggestions. My 12 yr old, desexed male, insists on using the tiles in my house as his toilet area (only urinating). He lives in the house with another elderly female, however has access to get outside 24 hrs per day. The problem appears to be worse when it is raining, but irrespective he does it anyway - never when I am around though. When outside he will often urinate on hard surfaces in preference to trees, shrubs and grass. I am at my wits end