Before I start, the only way that you will know for sure is by having your son assessed... and that since autism is a spectrum, not everyone experiences it the same way so some autistic children will show some indicators but not others... but from what you've said:
"no real pretend play (pushes toy cars around, sometimes says "stop" and "go"), likes letters and numbers"
Check if he repeatedly lines up his cars or any other toys, as lining toys up is one of the indicators of autism.
"Doesn't wave, say "hello", "goodbye" or "yes" or "no""
He might just be shy, see what happens if you encourage him to start doing this
"stimming - it's come & gone over recent months, but at the moment consists of head nodding, hand flapping and humming"
Head nodding, hand flapping and humming are pretty strong indicators of Autism - but he is only 3 years old so he might just do it because it's fun? I think that stronger evidence would be if he did these behaviours in association with some kind of anxious situation
"runs a lot, very active, tends to run rather than walk"
Seems like typical 3 year old behaviour!
"when taken to the zoo or a child's themed outdoor park, he will just run about like a headless chicken, won't take much interest in what is there"
Again, sounds like a typical 3 year old!
"language delay - does say short sentences, usually the same ones, usually asks "what's that" when out repeaedtly (mother speaks Polish to him and I speak English to him, so bilingual household)"
Repeating the same sentences over and over again is an indicator of autism, but he is only 3... does he repeat what other people say often? You could start using a word that is unfamiliar to him often and see if he starts trying to repeat it... "as a matter of fact" is a good one to try.
-he sleeps well
This could well be because of how active he is, and it is interesting that he hums himself to sleep, but you did spend many years humming to him as a baby so he might just be using that technique on himself!
-eats well and eats pretty much everything given to him
See what happens if you give him a meal where things on the plate are mixed up or touching each-other, especially sauces. Obviously try this with food that you know he likes!
-good eye contact
Bad eye-contact is a strong indicator of autism, but of course it is a spectrum so not all people experience autism the same way...
-not obsessed with anything
Again, this is usually a strong indicator but not everyone with autism does this
-generally compliant, will follow basic commands
Even if a reason isn't given?
-likes nursery
Schools and nurseries have gotten a lot better at being understanding and non-judgemental of children's different behaviours, so he could just like nursery because they are catering to his playing and social needs / style.
-recgonises emotion and shares emotions
Beyond happy / sad / angry ?
-likes hugs and will ask for hugs
That's nice, you could check for touch-associated indicators by giving him a cotton-ball 😊
-seems OK with change of routine
Getting anxious over a change of routine is usually a strong indicator of autism