It could be that your cat has a medical problem such as cystitis that is causing discomfort, so you really should bring him in for a check up first.
If that is all clear, it could be a behavioural problem. Cats are by nature solitary animals and often very sensitive about their personal space. If you have several cats living together, even if they appear to get on well, they may well be subtly competing with each other for access to food, water, litter trays, sleeping areas (especially high vantage points) and points of entry or exit from rooms. Apart from the obvious signs of irritation such as hissing and general light combat, one of the classic ways they may express their frustration is by territory marking, and the front door mat is a common choice.
Make sure you provide more than one litter tray so they don't have to share one, or even worse, use one that has already been soiled. Scoop regularly, change the litter daily and scrub the tray weekly. Cats can become fussy over litter and smells, so experiment with different litter types and try an entirely new tray.
Give them a choice of feed & water bowls, not all grouped together, and allow them to feed separately. Given the choice, most cats like to graze feed as often as 10-20 times a day.
Provide a selection of raised sleeping places from where they can survey their territory.
And try using a feline facial pheromone diffuser (http://www.feliway.com/gb) to help calm reduce their stress levels.