him - the mystical David (Eze 37:24, Eze 37:25; Jer 30:9; Hos 3:5). Given by God (Isa 49:6).
witness - He bore witness even unto death for God, to His law, claims, and plan of redeeming love (Joh 18:37; Rev 1:5). Revelation is a “testimony”; because it is propounded to be received on the authority of the Giver, and not merely because it can be proved by arguments.