I do have one question, however: How does the swallow-abilty work-I get it that he swallows you, but how am I supposed to escape the grab while I am restrained? I can`t move, thus I cannot escape the grab by mundane means. Even if I were able to teleport I could not get out since that would require Line of Sight which you specified I do not have while inside the behir. 
And one more question: How often does he have to sustain the grab? On every turn? Or maybe on initiative count 20?
Also note that this power could lead to a a fast TPK, should he ever be able to swallow the leader of your party. No LoE to anything outside keeps him from doing anything other than maybe sustaining powers such as Moment of Glöry. In a worst case scenario you are practically taking him out for the whole fight and Solo battles and leaderless parties don`t mesh well
I think you should specify how to escape that 'grab'. My suggestion would be to turn into a skill challenge, something in the line of: 6/3; Athletics, Acrobatics, Endurance with aid another actions for the people outside triggering from Nature, Arcana or Heal. Make the aid another actions Minor Actions against a moderate DC and make the checks for the grabbed PC challenging, e.g. Hard DC.
Another, easier approach would be to allow focus fire on the behir`s stomach: Give it resist 10 all damage and 80 Hit Points and include a negative side affect for the grabbed PC, I would personally deal 5 damage to a grabbed PC for each succsessful attack on the behir. Once the party manages to destroy the scales behind which their friend was held captive the formerly grabbed PC can shift into an unoccupied square adjactent to the behir as a free action. Of course the behir should still retain his ability to swallow targets and the damage dealt to the stomach should not be distracted from his total hit points, just to keep things challenging 
But really, I like this monster, would you mind if I borrowed it for my games?
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