Hello icepole just about to go to bed, bit of a late bird tonight. I hope you found his teacher friendly and approachable, now you have expressed concerns and she has told you about his unwillingness to talk to adults, you at least know she is in the picture and can get regular updates. I would expect her and any classroom support staff to involve and encourage him.
If he has older or younger siblings, either you could ask older siblings to "big up" starting school, or if he has a younger sibling, point out that they only do X, and that would be even more boring being home all day and doing what they do. Especially easy if you have a baby or small toddler, without denigrating them, just point out what a big clever boy he is, etc.
(My DS at about that age expressed concern I'd be lonely, I felt guilty imagining the hours he'd spend at school while I filled my day with his younger sister and "me" time!).
Is he "scared" because it's all new and loud and unfamiliar or has he mentioned anything specific? My DS was an observer rather than a joiner-in, he much preferred getting on with his own thing. My friend's DS found it all "boring", actually he was rather worried about going to the toilet and that played on his mind.
Has he mentioned even one activity he does like at school, even it's playing outside at break time? Then at least he has that x number times a day. Did he go to pre-school, does he know other children by sight from the neighbourhood? Has he friends in that class? I would bribe in the nicest possible way to get him to go in willingly by holding out the promise of a small treat when it's home time.
I imagine you already coax him, to give school a go. Add his teacher and classroom assistant or whoever like him and there are nice boys and girls who will like him too, it's all right if there is only one thing (whatever it is) he likes best. Can you say to him, lots of children in P1 (sorry force of habit, am in Scotland, is he in Foundation class) don't always enjoy much, to start with, but everyone has to go, all children do, and it gets better, (especially just before Christmas).