How long have each of you been looking for work in the UK? If longer than 3 months, i definitely wouldn’t turn it down without looking at all of the options and costs.
How did this job offer come up? Presumably your husband applied and interviewed for it if he’s been offered it - so why didn’t you discuss all of this before he even applied? Even just a basic chat about what you would do if he got the job! Did he only apply because he’s receiving whatever the jobseeker benefit is these days and he has to apply for a certain number of jobs and he didn’t actually expect to be offered it? If so, would declining it affect any benefits you are receiving and can you afford for that to happen (I am fortunate enough not to be in a position where I know how that all works, so please excuse my ignorance if it wouldn’t be an issue).
You seem confident that your son would only pay domestic fees for university. How do you know this? Is one of you Australian so you could move there as permanent residents (I’m assuming spouses and children can do this without issue?) or do you already know the type of visa you would be getting? Have you checked and confirmed you will be eligible for visas and the right to work there?
Can you afford to live off just your husband’s salary in Australia? If not, will you have the right to work out there? Will your son?
All of the above I’d have researched and asked about before he applied for jobs in Australia.
You mention living apart from your son isn’t an option as you’re a close family. But it also sounds like you’re anticipating he will go to university - so in 2 years time he’d be living away from you anyway. How long will it take to get visas sorted out and move? That would reduce the time down, so maybe look at options for your son staying behind. Do you have family that would be prepared to take him in
whilst studying A-Levels maybe?
I would definitely discuss the whole topic with your son. He’s at an age where he should be involved in decisions that will affect his life. Would he be wanting to come back to the UK for university? Would he be happy moving to Australia and living out there permanently? If you all moved out there and he was unhappy so decided to return home when he turns 18, how will you feel about living on the opposite side of the world to him? Or conversely, what if you’re out there for a few years and you two decide to move home but he decides to stay?
How solid/happy is your marriage? If you were to separate and divorce, would you have the right to stay in Australia? Would you still have the right to work there? Could you afford to? Would you be entitled to any benefits should you need them? Have you ever in the past discussed emigrating to Australia?
How soon does a decision need to be made? I presume not long - and you have a lot to think about and discuss.
Good luck - to be honest, from your post, I get the impression that you don’t want to move there and are looking for reasons to block it, but your husband would like to (hence applying for a job there). If that is the case, you need an honest discussion with your DH so that he knows that this is not an option for you - and if that’s forever it just until your son has left home for example. If he’s Australian and homesick, you may have to be prepared that he will choose going home over staying if he thinks you will never want to move there though.