Have you searched his name in LinkedIn before? If not, there is a chance that he has searched your name for some reason.

I once heard that some dentists would check patients LinkedIn profile to estimate how much they should charge them next time. And some people are just creepily curious. They look into your online profiles for no reason.

I don't use LinkedIn's mobile app. But, if the app has permission to read your contact numbers, it could be simply that.