When you go insane or are knocked unconscious, you lose half your items(including retainers):

For the word items, does this include

Clue tokens?

Skills?

Gates?

Money?

Monster trophies?

Thank you

No on all counts. Only common, unique and spells, though I've heard somewhere the rail pass counts and would assume exhibit items count too.

And so do the Deputy Revolver and Patrol Wagon.

You lose the half of your items and clue tokens.

You've got also the choice of take a Injury/Madness card instead, and don't lose anything.

fairly certain he doesn't have any of the expansions yet, so no injury or madness

A note on Injury/Madness: the DH rules say:

When an investigator is reduced to 0 Stamina, he may choose to gain an Injury card. If he does this, he does not lose any items or Clue tokens for being knocked unconscious.

Since Retainers are not items, I assume that you still lose your Retainer even when you choose to take an Injury or Madness card.