There are two aspects on which to attack - technical failures and human error
A strip of sticky foam plastic round the door, to seal off the escape route? He would have to be sure to always close the door on the pong for that to work - give him some firm training in that respect.
Also train him in always closing the loo LID as well as putting the seat down. Or in using the upstairs loo for potentially smelly visits?
I think there may be extractor systems which draw air directly from under the seat/lid, thus minimising the chances of the air in the rest of the room being contaminted, but I don't know chapter and verse for that. It probably would only be effective with the seat and lid down, so again human co-operation is needed.
