Manual Registry Hack

Open up regedit.exe through the start menu search or run box, and then browse down to the following key (create the key if it does not exist)

HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\Explorer

Create a new 32-bit DWORD value on the right-hand side named NoViewContextMenu with one of these values:

• Enable Context Menus: 0

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Symantec Endpoint Protection 11.0 compatibility with 64-bit platform
http://service1.symantec.com/SUPPORT/ent-security.nsf/docid/2007022310384648

It is by design

Q: Are there any limitations on the AntiVirus and AntiSpyware on 64-bit platforms?
A: These components will work as expected with the exception of right-clicking on an item and choosing "Scan for Viruses...". Right-click AV/AS scanning is disabled on 64-bit platforms

Ref:Symantec Endpoint Protection 11.0 compatibility with 64-bit platform

Any way there is some product enhancement request is present in the Idea section of this form.As per that this feature is in development.You can also vote for that so that in near future we can expect this feature
Context menu entry for "Scan for viruses" on 64-bit platform