Smells - look away now if you are easily offended!!

My loo is also next to my front door but I do not have that problem- I do not add smellies etc except maybe spray cleaning stuff down, leave a couple of minutes, flush and leave.

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.

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I believe I have cracked this and I swear by these -
Aesop Post-poo drops (or, more eloquently in French - Gouttes Anti-Odeur de Merde !! ) Not cheap - but very good, you could try Amazon or Space NK unless you have an Aesop shop near you.
Also their hand wash and for the loo itself I also like WaitroseEcological Watermint and Sage toilet cleaner which smells lovely!

Have you got an outside window you could him to open?

If not open the front door after loo has been used and leave open for 5 mins that should cure the problem!