Hide Whatsapp profile picture from a certain user
The only way would be creating a group with just you &amp; him, and then blocking him.. You'll not be able to message him directly, but only through that group (ofcourse he'll know you've blocked him).. 

Another way would be starting a conversation between you &amp; him and deleting his number (yes, it's mentioned in the question), but you'll still be able to chat with him (his number will be displayed instead of name) and make sure 'Who can see Profile Pic' is 'Contacts Only'..

Other than this, I don't think there's any other way..