In the original Expanded Universe stuff (called Legends now) he escaped and was found nearly dead by Dengar who helped nurse him back to health.
In the current canon he has never definitively shown to be alive, but it is heavily implied that he escaped the Sarlacc because a guy on Tattooine named Cobb Vanth ends up buying some Mandalorian armor from Jawas. Heavily implied to be Boba's and somehow out of the pit when the Jawas got ahold of it.
Also, there are rumors than Morrison may be pulling double duty and also playing Rex, possibly showing up with Ahsoka. I'm actually more excited at that prospect, although Fett could be interesting.