Oh a big, virtual "hug" from us. As far as I can tell, nearly everybody I know is/was/will be in Devon this Easter. Our town is virtually deserted!
DD knew nobody in reception..fast forward to yr3 and we were newbies again....
Both schools have had class reps (either for school stuff or PTA activities). Somebody dashed around the playground and PTA things to get contact details at the first school (and then the school office organised it at the second - it was fine to opt out).
I think I made my first "friend" when I/somebody else took a photo or video clip including DD that I asked for (so at least 1 email address shared). That might be more of a challenge with an introvert child.
In yr1, I am guessing that you are still there to do school pick-up and drop off. Can you and your son linger in the playground so either you or your son get chatting to another family (take some fruit and water and they'll all pounce on you!)?
Possibly too early for you at the moment, but Minecraft is massively popular and we've had many a successful introductory playdate where the children share their "worlds" with DD. They sit jabbering away to each other and giggling.
Once they have a common interest.............
No, it won't work all the time, but I bet many younger siblings are playing it by yr1.
BTW, I tended to choose the parents I liked and persuaded/coerced DD to play rather than wait for suggestions at that age. These days I find myself dashing up behind to make the introductions!