Since your client is in the business of Trading and hence he will be lieable for PT.
Click on the following link : https://mahavat.gov.in/PTRC_PTECRegistration/
Fill up the details and select PTEC Enrollment. (PTRC will be applicable if he has employees drawing salary of Rs.5000/- per month)
On the next page you will get a REQUEST ID.. DO write it down somewhere cause if that is lost then you wont be able to do PT registration and you may have to ask the officials to recover your Request ID.
Fill up all the details. (If your client is eligible for VAT then do get him registered for MAHAVAT) and then you can submit the form and on the next page you will get the confirmation of when you have to go PT Office for the Registration. Also do take out print out of the file given in the link.
In case if you want to fill up details later, then Just save the form and do not SUBMIT it. And next time Use the PAN No. & Request ID to see your saved form via the above link.