"When I left in May I didn't realise I wouldn't be going home..."
I think you just answered your own question.
Also, time spent in a relationship means nothing really when it comes to passion. If it's right, it's right! I moved to the UK from Perth, WA and less than a year after being here, I met an Aussie guy (from Perth) online, and he's moved to the UK to be with me while I go through the PR process. I'm not trying to make this about me (I promise!) but it does just go to show that you should go with your gut. What your gut and heart are saying is obvious.
If he is prepared to go, what's stopping you? I suppose my only advice would be to ensure your relationship is well-documented if he is going over on a WHV. If you plan to live together, keep a record of rental payments, etc. This is just in case he doesn't end up with sponsorship during this time, and you decide to continue the relationship. Hope this makes sense!