This is completely out of line and quite childish. He needs to know that face-to-face socialisation is de rigour for such events.

Insist that the tablet remain home. Insist that taking it is inappropriate. Ask him to consider: Does anyone else behave accordingly? Perhaps come to an agreement of under what circumstances during the evening is he stressing so much under that makes it necessary to leave and go home (it sounds to me as though having this tablet is a security blanket of sorts; a coping mechanism).