He cannot, however, it may have happened that he has put you in PIP and not informed you. A formal consent is not sought from the employee to put him in PIP, nor you are required to be put sign any documents for that. HR would have been informed, you can discuss with them as well.
PIP means you are not doing well in your job. You need to be careful as if you don't pass the PIP stage, you could be fired from the job. I will suggest you document the fact that you were not aware you have been put on PIP. Also ask your manager to share key improvement area's which he is tracking as a part of PIP. Also ask him the duration for the same and set up a regular meeting to ensure that your progress is recorded and tracked. Ideally your manager should be doing all this, but looking at your question it seems he has not done a good job communicating or managing you PIP.
I will also suggest to take a look at your performance and see if putting you on PIP is justified. If there are performance issues, either fix them, or look for a new job.