Dikembe Mutombo flew an 8-year-old boy from Africa with a large tumor on his face -- who's been shunned by society -- to Los Angeles ... so he could be operated on by a team of American doctors.

TMZ Sports saw Mutombo at LAX ... when he told us the heartwarming story.

The NBA Hall of Famer says he met the young boy back in September at the hospital he built in Congo ... and immediately knew he wanted to help.

"I was touched by what he's going through as a young boy who's 8 years old. Not going to school. The way he's been pushed away by the society," Mutombo told us ... "His mom has to keep him in the bedroom every day because people are talking bad about him."

Dikembe -- who flew into L.A. earlier today -- was waiting at the airport for the boy's flight to get in from Africa ... and he invited our cameraman to watch the reunion.

Now, Mutombo set up the young boy to be operated on by Dr. Osborne in Beverly Hills for free ... before he returns to Congo where, the hope is, he'll be able to return to school and live a normal, happy life.