I also have a child who seems quite happy not mixing, but we just had his 4th birthday party with 8 children, and it was a success because of the storyteller we hired. If you are in Herts or N London (or Beds, I guess) then I will happily put you on to her. The children 'got it' really quickly and she also did a little bit with puppets, which they loved. We started off by making and decorating cardboard crowns to wear, and because the parents were good about joining in, it didn't matter that 2 of the children didn't want to make a crown.
ds suddenly seems to be coming out of himself, maybe coincidence that he has started state nursery 3 afternoons a week (he has also been in a private nursery since he was 9 months old, always played on his own), but it has been really hard to watch him be so bad at mixing, and AWFUL at sharing and taking turns, because I know how dreadful my own shyness was as a child, so I do sympathise with your worries.