Movie or tv series from the 90s (saw on television, in English, and in color) about these people who discovered portals to different worlds on Earth. A (teenage) boy accidentally travels there, and meets sorcerers and people who can turn into animals. He has to fight off the bad sorcerers and close the portals, so they don’t find a way to Earth and take over.
“Crossworlds”(1996)?
http://www.imdb.com/title/tt0115985/?ref_=fn_al_tt_1
Sliders
Neither one of these
This isn’t a movie, but a short-lived television series. Instead of portals to other different worlds, could the teenage boy have been doing some time travelling? I do not remember this series well, and honestly, I’m not sure I ever saw all the episodes, but your description reminds me a bit of “Josh Kirby…Time Warrior!” Goodness, it was cheesy and some of the dialog was astoundingly poor, but they were fun.