That’s hard to diagnose from here, sorry. My assumption is that he can log on to O365 but can’t log on to Project Online. If that’s the case and he is assigned a license for Project Online then it should work. If he can’t log on to O365 then that’s a bigger issue. Sorry that this is so basic. If he is setup just like everyone else and still can’t log on then I suggest submitted a Service Request to Microsoft (within the Admin center).