Just to give you hope - when we moved to our current town, DS (under Ed Psych in his old school for severe anxiety, and a school-induced selective mute at his old school also) joined a year group of 58 in which he knew not a soul. To make it worse, he actually started 3 weeks before the end of Year 1, and nobody had joined that year group since the beginning of Reception so they were a well-bonded group with very established friendship patterns.
Equally, DD joined the Reception class of the same school in the September, again knowing precisely 0 out of the 59 other children in her year, having spent the previous 2 years at a pre-school where we used to live.
Both quickly developed an extensive friendship network (to the extent that other mums, and to a great degree the school, didn't really believe the anxiety / selective mute facts about DS, they thought I must be exaggerating) and i would say that neither suffered in any way from being in a class of children they did not know when they started.
So think positive! You and your sister should emphasise that all the other children are new too, that you know that he will make great new friends, that he will also get to see his old friends at playtime so will end up with twice as many friends in total - if you make it sound a positive and exciting adventure, you are doing a much better thing for him than by getting involved in appeals etc.
Of course you and your sister will be anxious. The hours of DS's first day at his new school were some of the longest of my life (he emerged shrieking across the playground 'My new school has 3 BREAKS and ALL MY CLASS IS MY FRIEND' ... this from a child who did not speak to anyone except immediate family for several months was just amazing!) but don't let your anxiety show to the child and they are likely to be much more resilient than you expect.