I read this book 15 years ago or so and am trying to find it again. A teen-aged boy transports through time, I think via a fountain, to a world which he doesn't recognize (comes to find out it is earth hundreds, if not thousands, of years in the future) When he is first transported he comes out in a jungle type area. The book goes on to have him captured and then enslaved by a clan. He eventually wins his freedom by acts of valor in battle. He becomes a major warrior in the clan and falls in love with one of the girls. Once everything falls into place with his life there, he is chasing an enemy in that same jungle and I want to say a beam of light transports him back to his own time without him aging. He appears at that same fountain wearing his battle gear about to throw a spear with everyone in the mall area wondering what he is doing. I'm pretty sure this is fairly accurate, but it has been a while and I could be misremembering some events. Thanks for your help!