Source: The Dream Page - thanks to Treggy !
LUKE BROWER The 7th and the last (91/92) season of this popular sitcom saw an unexpected addition to the Seaver family. Mike, while teaching at an urban high school, encountered homeless Luke Brower. Luke---more or less in Ben's age range---eventually became a full-fledged member of the family, even taking over Carol's room when she left for the dorm.
Mr. Leonardo DiCaprio -- then only 16 years old -- joined the cast of Growing Pains when the show felt their ratings were dropping. The Seavers, reluctantly at first, took in a homeless boy, Luke Brower. Luke was one of Mike's students at the Community Health Clinic and quickly became one of the Seavers. Luke left with his father, George, during the end of the final season to help him open up a truck stop in Tuscon. The sitcom ended before Luke ever came back; Maggie got a job in Washington D.C. which required the family to move. In the last episode, the Seaver family gathered around a picnic blanket on the floor of their empty living room, remembering all their pastimes.
Here's a quote from Ashley Johnson who played the youngest daughter in Growing Pains -
She remembers Leo as a "goofy boy".
"I didn't have a crush on him then but now I do! I saw him a couple of years ago and my hands were shaking. I said "Hi, there Leo DiCaprio", and he jumped up and yelled "Ashley!" and gave me a big hug. I didn't want to take a shower for the next four days!"