Everything (as in everything) that takes place on the internet today is recorded and traceable. That is, the FBI and properly empowered local police can access the federal database that captures EVERYTHING.
Emails are particularly easy. You don't even need to go through the feds. you can get those directly from the ISP and/or the web-based email company. They have entire crews who respond to warrants.
If he downloaded porn, it is traceable. If he uploaded files to others, it is traceable. what the actual files are may not be identifiable, except for the porn he would have downloaded to his own PC. The FBI is quite adept at this, and some locals have been trained. My local CAC hold those classes for law enforcement.
Imaginary friends are people too!