I have a boy just like this in my kindergarten. Painfully shy is a great wording. With plenty of support he will actually talk to the group, but it often ends in tears.
Anyway I have no great suggestions but would love to know what others come up with. I have just been taking baby steps with this boy. For example he wouldnt do show and tell but would happily ask questions if it was something he was interested in, so we started there. Now he does show and tell but doesn't say anything much apart from introducing himself.
Glad to hear the update on your oldest. Wonderful. What a great head start you gave.