If opponent does action , and I do nothing in response I pass my action turn, can he do another action or does combat start

Combat does not start — he can play another action. Passing in response to his action doesn't end the showdown by itself.

Here's the flow: his Action resolves, then Focus passes to you (Rule 346). You get the next opportunity — play something or pass. If you pass, Focus goes back to him, and he's free to start another action chain. The showdown only closes (and combat proceeds to the damage step) when both players pass consecutively without either playing anything (Rule 348, FAQ #2268).

So the showdown keeps going as long as either of you keeps starting new chains — your single pass is not enough to force combat.