I can't contact him directly in Linkeding, so the burden of proof is on your shoulders. Make him link this project on his profile if he is a real member. Or maybe the reason for not linking is that he doesn't want to get associated if you get
sued I think if he is really a member of the team then there will be no problem to confirm this participation in the team.
Very often people are team members of many projects but they do not put all the information in Linkedin.