Sorry to the OP, I've been unable to post my reply due to a somersoft tech bug.
Congratulations to your son, but he doesn't know what he is in for and I can briefly convey my experience.
By all means help, but I would suggest he not be tied down with commitments. Furthermore, with the upfront payment of university fees attracting a significant discount (25% from memory when I went through), is this perhaps more helpful?
With regards to your sons placements, he has little power and no capacity to predict where he will go. I was also at UQ, where the first couple of years lectures were based. In my first year however, i was also sent to Bundaberg and Goondoowindi. After that, I went to Toowoomba, Logan, Royal Brisbane Hospital, Quilpie and Amsterdam!
Furthermore, after medical school the travel really begins - I've lived/worked in Cairns, Ipswich, Logan, Gold Coast, Southside Brisbane, Melbourne and England.
As a student, I always worked a job, but had little capacity to increase earnings, moved out and back in to home multiple times, and could not service a $19000 loan without undue stress.
The other consideration is the opportunity cost of him having a fixed abode - the placements outside of Brisbane were amongst the most rewarding and served me on a rapid path to training, specialisation and subspecialisation. i would not have chosen any of these peripheral posts, but was grateful in retrospect - had I good reason to stay (like my own house), I would have fought to stay in central Brisbane, and this would have been a mistake.
By all means buy the house, and let him pay rent to you. But in my opinion, allow him to be competely mobile and discourage him from borrowing of any form.
His investment from here on in needs to be in his career - what is he going to specialise in, he needs to think about it. He needs to start research in that field, submitting abstracts, writing papers, studying. THe process is only getting more and more competitive. Surgical training for example, is hardly accessible without multiple publications, and subspecialty training hardly accessible without a phD.
Good luck to you both. Hope that provides some help with the big picture.
(PS Medical school was the greatest time of my life, let him enjoy it).
B.