The advice on this thread is all excellent.
The only things I'd add/emphasise:
Re. becoming fixated: will he be devastated if he doesn't get in? If so, be careful, because that's quite a likely outcome for any applicant.
How will he feel if he's not top of his year at uni? What if he's middle of the pack? Bottom third? Bottom fifth? (Just to be clear, nothing you have said makes me think he's not up to it. I'd ask this of any applicant. A fifth of students will be in the bottom fifth. Would it crush him?)
Will he enjoy (or at least manage) the workload? Some students thrive on it (like an earlier poster's son), others will not enjoy it. Even if he's excellent, does he want a big workload or would he rather have something a bit lighter?
If the three above are all good, then I think you have very little to worry about.
Autistic: this is pretty standard!
Cost: Oxbridge not necessarily more expensive (unless you are comparing it to living at home). You might want to consider one of the richer colleges as they tend to have more hardship funds. Once you are there, most colleges will work hard prevent anyone dropping out for financial reasons. (I think many publicly say they want to ensure this never happens. Even if that's not publicly stated, if you go to richer colleges it is certainly the policy privately once you are there).
"Super-curriculars": watch Matt Williams's YouTube videos on this (Jesus College, Oxford). Oxbridge put zero weight on irrelevant extra-curriculars (music, sports, DofE, etc), no matter how impressive. But they do care that their applicants really care about their subject intellectually, and can prove it. So things linked to his course or his intellectual passions: yes they want this. There are many ways to demonstrate this (watch the videos). Perhaps Google "super-curriculars for engineering". It doesn't need to be things that cost money, it just needs to be sincere and material and driven by your son. A good college will make sure they aren't accidentally privileging people who can afford expensive super-curriculars.
Good luck!