stinky poopin cat!

try mixing his poo into the litter in the tray and perhaps keep him confined with the tray, in a large crate, like a dog crate. Spread his bed around the rest of the crate as he won`t be pooing on his bed. He`ll soon get used to the tray

He must not like the litter in the box - try a new litter again and you may not have any problems - what litter have you tried?. Cats dont generally make a mess in the house unless something is wrong or the box is in the wrong position - something may worry him about the positioning of the tray unknown to you - they like a quiet spot not a busy spot. The other thing is get another couple of boxes too. I use one with a hood - its very private for them too

when we got our first cat, we kept him in the kitchen unless we were in the house, the kept all the upstairs doors closed. he evetually got used to his litter tray in the kitchen. he will actually go and play outside and come in to use the litter tray!!
also scrub where thy have been to the loo to get rid of his scent!

Nothing worse is there - especally the smell of cat pee !
I agree what the others said about keeping the cat confined to a small area ( just one room is best ) for about a few days with access to a litter tray.
However there is one other thing you need to do and it might be a bit costly. The cat will keep using other areas that have been pooed on every now and then ( say for instance he has pooed in the downstairs hall from a cats point of view why bother trekking upstairs to your litter tray when the area you pooed before is handy ? ).
It might happen infrequentily ( when the cats a bit stressed )or it might happen a lot but until all the smells of where the cat has previously pooed are taken away the cat will just get confused and go back to these areas. Unfortunately there is no rela way that I know to take the smell away , so its usally throwing out anything soiled ( which means carpets too- sorry. in the past I have just cut out areas of the carpet ).
However if you confine him to a small area and take away/ clean all areas that have been soiled you should be ok. I have just got a cat that has been outdoors andliving rough and using these ways hes now very good with his litter.
Again with the litter nothing highily fragrenced or colured- some people suggest putting some dirt from outside in too.
Good luck !!!

To eliminate the smell of cat pee/pooh, I find that cleaning the area as normal then sprinkling with a fair amount of bicarbonate of soda and leaving for a few days, then vacuuming, usually gets rid of the smell.