Can Boba Fett fire a secondary cannon weapon (HLC) out his rear arc?

Nope.

Thank you.

Nope.

As Panzeh said - the short answer is Nope.

The longer answer is as follows:

(New) Rules Reference Page 5.

AUXILIARY FIRING ARC

Some ships have an auxiliary firing arc identified by dotted lines printed on its ship token. These ships also have a unique primary weapon icon printed on their Ship cards. When attacking with its primary weapon, a ship with this icon may attack an enemy ship that is at Range 1–3 and inside its standard or auxiliary firing arc

When attacking with a secondary weapon, the ship must still target an enemy ship inside its standard firing arc (unless otherwise specified on the Upgrade card).

Note also that while (with one exception) secondary weapons may not be fired from an auxiliary firing arc - it is still a firing arc for any other purposes. So game effects that trigger from in / out arc conditions ( Autothrusters, Tactician, R7-T1 ) still apply.

As Panzeh said - the short answer is Nope.

No would be an even shorter, but still correct, answer