After Many Many Many Rumors and Then being true! Shadow the Hedgehog was not confirmed to be playable for Sonic Boom. Today SEGA announced at Pax Prime That playable demos of Sonic Boom will be playable. Someone also asked about Shadow the Hedgehog. Shadow was not confirmed to be playable according to SEGA.
This was told on the comments of The Sonic the Hedgehog OFFICIAL Facebook page. https://www.facebook.com/Sonic/photos/a.406270927417.185036.23050342417/10152628829587418/?type=1&theater
Sorry to Disappoint. Sorry that also i got wrong information ^^;.