They can hack his computer before nabbing him, grab tons of his data from connected providers, get a warrant to sneak in and plant one of several type of bugs that would allow them to aquire his pass phrase. Burst in while the machine is on and aquire the unlocked data or get the key from ram.

Failing that they can convict him based on other evidence or at least convince him that they can and cut a deal for access.

In the vital cases which are normally cited as examples of why we must allow cops to violate our right options abound.

What's left is petty crap and police incompetence which serve as poor justification for giving up our rights.