Daz, as a current serving engineer in the Army with 15 years service, I might be able to answer a few questions for you.
Yes, your son can join the army and do an engineering degree through ADFA (Australian Defence Force Academy), all paid for. After that, he will have to spend a further year at RMC (Royal Military Academy) to finish off becoming an officer. After that, he will go to the School of Military Engineering (SME) to complete another 8 month course becoming an Engineer Officer (Basic Trade) and learning how to do all the basic engineer stuff that his solders will learn but at higher level. From there, he will be posted to a unit (1 CER - Darwin, 2 CER - Brisbane or 3 CER - Townsville)
What Engineers Do? Well, we are the jack of all trades, master of none! We build bridges both over rivers and rafts, minefields, do demolitions, make fresh water, drive plant equipment, draw maps, do search (afghanistan), construct buildings (chippys, plumbers and electricians), drive vehicles and work dogs.
My skills, though very specialised, are transferable to other jobs, like so many within the engineer corp, but in most cases, there is a return of service. Just depends on what you do.
If you want, PM me for more info.