not responding

I filed for divorce, I did tell him I had done so. (I had told him a few days earlier that i wanted us to divorce, he was not on board with the idea).
We did receive a copy of petition in the post. I handed him the letter.
On a number of occasions I have asked him if he had responded or opened the letter or received the email his answer is always he has not received the email and has not opened the letter. The period for responding has already passed.
I would like to go forward but before using a paying service to serve him the papers directly, can someone tell me what can I do?
Is sending a what's app message to ask him about responding to petition and if he has finally opened the letter a good idea? Has anyone done this and what was the outcome? Any suggestion?
If he was to answer would it be proof enough to present it as evidence? Or a read tick enough?
Thanking you for useful answers, sharing your experience and advice. I do not want to threat him but to get him to simply respond to petition.

I had a similar situation where STBXH wouldn't respond but acknowledged he had papers, I applied for Deemed Order of Service with the court, £53, the What's App message were used to approve the service.

thank you for your answer.
can i ask ?
was the question similar to : have you responded to /or received the divorce petition? and his answer was yes
or a bit more detailed or if different if you don't mind telling me?
I'm still getting my head around these procedures, what do you actually mean by : 'I applied for Deemed Order of Service with the court' ?
Was it because you had proof he had acknowledged he had the papers (also did he mean read or received without reading the email or the posted letter or it did not matter?) .
I would appreciate if you could get back to me as I believe there are important details. Thanking you in advance.