He should go to the court immediately and go to the clerk's window to fill out an Answer. Part of the Answer should claim that service was improper (you may or may not have this correct, based on your posting, it could be what is called "nail and mail" service that was left at a door) but you should preserve that in your Answer. He should also address the merits of the claim in your answer. Do not wait, as you do not want a default judgment against him. You probably have 30 days from the date on the summons, but I still would not recommend you wait to go to take care of this. Even if they were not served properly, ultimately, the service would be effected so while that could delay the case, it would not delay it forever. It is probably best to address the issues in the case, and try to work out a settlement (if you owe the monies claimed) with the attorneys in Court. I wish you the best with you case. (Again, though, do not wait to go to Court and file the Answer).
Your post didn't indicate if you have a defense to the claim. Perhaps you could contact the attorneys for the creditor to work out a payment arrangement.
Putting in an Answer alleging improper service might be good in the short term, but making a deal now might save you a fair amount of money in interest that might accrue in the future.