Here's how you do it.

Take a syringe. Open his toothpaste. Put the tip of the needle *just* into the middle of the toothpaste opening. Inject. IMPORTANT: smooth over the end of the toothpaste with your finger so that it looks normal when he opens the tube.

He'll put the toothpaste onto his brush, not think for a second that anything would be wrong, and start brushing. By the time he tastes the ink, he'll have done a scrub or two onto his teeth. Which is really all you need.

If you're worried about your ability to not accidentally have the ink reveal itself when he squeezes the paste onto his brush, then buy a couple of practice tubes and practice with food coloring. (That's how this prank is normally done. Tattoo ink would really fuck his shit up, though.)