The reverse culture shock of coming back to Australia after being over there could be quite considerable. I know I've felt strange, disconnected and like I don't "belong" after being away travelling, and especially when I've been to a developing country where the difference is so stark. I suffer from depression and anxiety so I'm predisposed to these sorts of phenomena; something like this may have contributed to him going down a negative path.
Reverse culture shock.
You'll note that it states that if it is "involuntary", it is worse. He was forced to come home...
https://www.state.gov/m/fsi/tc/c56075.htm
From that link:
Voluntary versus involuntary reentry: involuntary is worse
Expected versus unexpected reentry: unexpected is harder
Age: reentry may be easier for older people who have been through more life transitions.
Previous reentry experience: the first time is worse.
Length of the overseas stay: the longer the sojourn, the greater the chance for adaptation; hence the harder it may be to leave and come home.
Degree of interaction with the overseas culture: the more involved you become in the local culture the harder it may be to leave it behind.
The reentry environment: the more familiar and supportive the easier the reentry.
Amount of interaction with the home culture during the overseas sojourn: the more familiar the returnee is with changes in the home culture the easier the reentry.
Degree of difference between the overseas and the home culture: the greater the difference, the harder the reentry.