In our game last night I attacked someone who had only two cards in hand, he chose to use one as reinforcements but could have used both. Would he have been allowed to use them both and if so what would happen on his turn since he wouldn't have a card to add to the Impulse?
According to page 12 of the rulebook you don't add a card to the impulse, so it remains at 3 cards long for the duration of their turn.
Adding a card to the Impulse is not optional. If your hand is empty to start your turn, skip that step.
Also on page 12:
6. Draw 2 cards, and trim the Impulse to 3 cards.
After the player's turn, they simply leave the impulse with 3 cards and don't trim it further.