The "front" of a tree is the side that you see, or the side that faces something, and it does not matter that the tree does not have a front. If your first thought was "in front of", then it is probably correct. However, there is nothing wrong with "next to".
"In front of the X" does not require X to have a front. So "in front of" and "behind" are somewhat unclear. In some cases they have meaning. For example, "in front of" a building usually means "on the side facing the street".
A path (where other people have gone) is like a street. So if he was running along a path, and he stopped where the path was close to the tree, then "he stopped in front of the tree" sounds good.
But "next to" and "by" and "near" can also be used, in almost all sentences.
He stopped [next to / by / near] the tree.