I'm thinkin YES, serve him the papers!! Some of these WS need that as a wake up call, nothing else does it. I got the "need more time" speech after I agreed to take him back. After all his begging and pleading for one more chance and me giving it to him, he then finds he just cant go NC with OW. Stupidly I let him stay in the house and gave him "more time". Two months later I am thinking he MUST be over her, and then wham, I see a page from OW on his phone. Come to find out, he was staying in contact with her so that he had a back up woman in case I decided to leave his butt.
Anyway, as one who has been there, tell him YOU need "more time" to go to your attorney and get those papers written up. Take the power back.