OP, YANBU to charge him rent if he decides to come home so you don't lose out financially but I think he should have the choice and your full support whatever he choses and right now, it does seem like you would much rather he doesn't come home.
I am in a similar situation to him, though slightly different because I did move out at 16 to travel and never moved back, partly because my family has made it clear they would rather I didn't come back. Frankly it is painful to know that your parents would much rather you were away, so yes I do make it work financially (because I have to, at least you are willing to help rent-wise, it hasn't been my case) but it is very stressful when some months are hard to know that I do not have the ability to potentially move back home to take a breather for a month or two (I would never want to move back long-term) to not have to pay for rent and actually think about what to do next.
I have been working non-stop since I am 16 which should make you happy, but I have had to deal with a LOT of crappy employers and abuse because I had no way out because I knew if I asked to go home, I would have a bunch of excuses about how it's best for me to stay where I am at and find another job (I guess one's mental health and sanity means very little to some).
YANBU to want your child to be independent and pay his way, but do not make him feel like you would much rather he never came home. I am also criticized for wanting to travel and not save for a deposit (though I am doing both) but traveling actually brings quite a lot of qualities and CAN bring clarity to people who are lost and don't know what to do.
I have learn to speak several languages fluently, I have lived and work in 7 countries and visited a bunch more than that, those language skills do look good on a resume, being confronted to actual poverty and lack of resources is also life changing and make people appreciate what they once failed to appreciate. It can really change someone positively and be a massive bonus on a resume. It's not a waste of time and if you are doing it right it's also not a "jolly" and can really give direction to someone who has none.
Finding work abroad, especially in Australia is not that hard if one isn't picky. I did go to Australia with 250AUD in my pocket so it is doable.
Your DS seems like he needs time to gain direction and confidence. He sounds a lot like my DB2 (18) who will work hard when in a job and never refuse to pay his way but have no direction and very little confidence in his ability to do things (right) and scared of being adventurous in general (he also chose not to take driving lessons because he is fearful of causing an accident, and is finishing baking school now but it's clear he only did it because he was told he should do it and he feels safer going with the flow and being told what to do instead of going for what he wants.)
I have booked him a flight to come visit me in a few weeks in the US because I think being abroad might give him some direction or at least motivate him to try and find his path or at least better his English skills.
There is nothing to be lost from traveling, and having a supportive family (which I know you are) comes a long-way.
I am conscious I will be working until I am at least mid 70's, I just wish I had had a little time where I could have been allowed not to have to worry about money.
Interestingly people who often wants 18yo to get on with life and start paying their way are people like my grandma (in her early 60's) who've spent a good decade being stay-at-home mums and then working part time and still managed to retire by the time they were 60 (I love her but yeah). I could be working full-time my entire life and still not retire until I am 70+ anyway , at 70+ traveling probably won't be the same as 20+ me traveling. Let him be young and enjoy it while he can, I say.