It looks like the stretching has stopped, so I'm guessing it's grown rather close to the light? That's good, that's the main way to stop stretching. Plants will tend to stretch when the roots start getting confined, as well, so you should consider going to a bigger pot. Do that, and keep it close to the lights, and that stem will thicken up, in a hurry. Also, an oscillating fan sweeping across them, will help, as well.