Sega announced via their official Sonic twitter account today that Sonic would be joining in with Lego Dimensions. He will be part of wave 6, an expansion pack, expected to release this coming September. Sonic accompanies 15 other franchises for this wave. In total there will be 30 franchises as of wave 6.
If you're unfamiliar with Lego Dimensions, it's an adventure game developed by Traveller's Tales. You can find out more about Lego Dimensions here.