We were in your position beginning of last year. We had until the end of Feb to arrive in Perth before our visa ran out and our then 19 year old son had been adamant that he wasn't coming with us - I managed to persuade him to come by saying that as parents we couldn't leave him, it would never sit well with us but he could leave us, he could come out for a few months and then return to the UK if he wanted. He agreed! He stayed for about 7 months then went back as missed his mates (just turned 20 then) he spent 6 months in a crappy job staying with a friend, spent all his money partying/concerts and realised he needed to be with us so came back!! He's giving it another go and is a lot more positive and I think a bit more mature.
It was still hard letting him go and so good having him back. It's a huge emotional rollercoster - it's not easy when their 18+ - good luck it will all work out, sometimes it's best not to over think things. Xx