He doesn't know how to interact with other children - he might never get invited to a friends house, or be able to cope with this anyway? Or have friends?
Maybe, maybe not. I'm autistic, I had precisely two friends at any one time during primary school, and at secondary school I suddenly had more people to watch and learn social rituals from, and I realised that I had skills that were very valuable in my new environments - as a result, and equally suddenly, I acquired a group of very close friends that I stuck with for the next 15 years.
He only eats 3 foods - so he'll never happily eat out, join a picnic or try foods on holiday?
That's very possible. And it may also be possible that out of nowhere he'll develop a special interest in food. Again, relating it to my experience, I had all sorts of trouble specifying the kinds of foods I enjoyed, and I very much restricted the contexts in which I'd eat around other people. Then, for no real reason, I decided that the best idea was to learn to prepare food to my exact specifications in order to save myself the hassle...which quickly became an exercise in optimising the process and the food, and as it turns out...I'm actually quite a good cook after all that, and I really enjoy learning how the science relates to the practicality of complex dishes.
He stims a lot - so will he be an outsider for life?
You need to understand that the world we live in now is a lot more accommodating of the nature of autistic folk than the one we grew up in (for me, that's the 70s and 80s, when any indication of not-normal meant being consigned to "special school" and forever being ostracised). He will have far more opportunities available to him than he would've done if he were born in any previous generation.
He only has a few words - will he ever be able to have an actual conversation? Or engage in life socially?
Even for children who're completely non-speaking at an early age, a significant number can suddenly and inexplicably start talking in whole conversations any time, often around 7-8. Communication development (actually, most development) in autistic children can be very, very lumpy - no progress for years, then suddenly catching up or even advancing past their peers, then stagnant again for a long time, then...etc.
Will I be his carer for life, with him living at home forever? He'll be unable to perform in school, and to get and hold down a job, partner, etc?
It's far too early to tell. It's a function of all of the above, but most of all...don't pre-judge the rest of his life at the age of 2. He doesn't know what's next, and neither do you. The best thing you can do is take it day-by-day with him, and develop your plan as he does.
Give him a quiet space in the house where he can retreat to recharge, learn to understand his triggers (sensory/social/etc) so you can avoid unintentionally setting him up to fail, give him the agency to communicate his needs to you without negative consequences, and don't try to force him into your way of dealing with the world - he's going to have to come up with his own approaches to all of that.
Do all of this, and you may be surprised at how well he adapts. It's not necessarily all catalysmic. He's still a person and he still needs to be given agency, he's just unlikely to do anything the same way you do. He will experience the world in ways you can't even imagine, and as mentioned above...that involves unbelievable joy and fascination as well as meltdowns and hiding from the world.