Toiletting on a door mat is classic behaviour from a cat insecure about the smells comming in, all kinds of aromas come in on your feet and they are strongest on the doormat, makes sense to him!
I think feliway to try and make the area less of a threat and id put a tray there, if hes doing it there anyway you arent encouraging him iyswim, if the feliway helps you can then try moving the tray inch by inch away and see if he follows it...