Escape to Grizzly Mountain.

Jimmy, a young boy trying to gain acceptance from his peers, is horrified when he sees a circus trainer abusing a bear cub. One of the circus employees tells Jimmy that there is a magic cave in the hills nearby that can transport people to another time, and that maybe he can use it to help the bear. Jimmy does some exploring, finds the cave, and is amazed to find himself transported back to the early 1800's where he encounters Jeremiah, a nature-loving mountain man with a bear friend of his own. Jeremiah agrees to come to the future and help out Jimmy, but the 21st century is quite different from the 19th, and springing the bear cub will not be as easy as Jeremiah thinks. Not in any way related to the "Grizzly Adams" television series.