Poor Snoball getting chased by Bing. You may have tried some of these but will throw them out.
Do they have a highly valued treat? Something they will all just about crawl over broken glass for but still safe for Snoball? If they do then try enticing them all together with it but only give it to them when they are being nice to Snoball. I had the reverse problem when I adopted Autumn she would charge my civies. But for raw chicken she would sit next to them all day. lol
Another trick is to trade scents. If possible switch up the litter boxes, trade around the unwashed bedding etc. I have even taken used dryer sheets to rub the gang down so everyone not only smell the same but also of home.
If Snoball plays try a play session with the civies first to tire them out some then have Snoball join in. A good time for this is right before food. Because it imitates their natural stalk, kill, eat, groom then sleep cycle. Plus in the wild cats will form hunting parties to care for the clowder. So by emulating that with play they should see him as a member of the family.
Mel, Maxwell, Autumn and The Fur Gang