Adam was formerly with Bath having joined their junior set-up in 2014 and then earning promotion to their Senior Academy in 2015.
The stand-off is a product of George Watson's College and Millfield School in Somerset and has represented Scotland at all age-grade levels.
He has played for Scotland U20 seven times and is the third Hastings to represent Scotland, following in the footsteps of his father Gavin and uncle Scott.