"Welcome." Sans even gives the higher trees a bit of a shake with his snout, to knock down a few more. "Eat well." He'll try to ignore the orange trees in future, at least, once they grow back. If nobody else wants the lemons...well, Sans still hasn't figured out why it's so important that he eat them.

Besides. It's nice to receive a reminder that there are still plenty of people to take care of Frisk, even with something as dangerous as Sans wandering around.