As this was served via solicitors, yours needs to write to his and give him 7 days to return the acknowledgment, failing which they will make an application for service to be deemed, which he or they (depending where the fault lies) will pay the costs of.
I am sorry, I assumed from your post you had a solicitor.
Ok - draft letter:
Re Your Client Mr X
I have contacted the court and been informed that the acknowledgment of service has not been returned. This was due on X date.
Kindly confirm the acknowledgment will be duly completed and returned to court by 24th June. Should I not receive your confirmation by that date I shall apply to the Court for service of the petition to be deemed and shall make an application therein for your client to pay the costs of the same.
If they dont respond to your letter and/or the acknowledgment isnt returned, you then make an application for service to be deemed. Let me know if that happens, and I will help you draft the application and affidavit.