This doesn't sound legal to me. How can you file lawsuit on me by the end of the week, but then give me 30 days to dispute.
It's perfectly legal. If he has already filed suit by the time you dispute the debt - and you're going to dispute it, right? - he will have to stop the collection efforts until the whole thing is verified.

If you know the debt is yours, call him now and make arrangements. If you're not sure, dispute it in writing and demand the original creditor's information. Be sure to send it certified, return receipt requested.