It’s not Eye of the Tiger (1986) or a Death Wish movie.
I think it was made in the U.S.
Maybe there were a lot of a scenery with trees.
His son gets kidnapped by the gang.
A medical doctor who tried to save the life of the protagonist’s wife or daughter joins him in his crusade against the outlaws. A female nurse also becomes a part of their team.
Later the doctor dies fighting the gang.
For some reason (vigilantism?) the protagonist is a wanted man in the eyes of the local police.
In one scene he ambushes two officers who are looking for him and forces them at gunpoint to undress and he cuffs them to their police car or near it.
The nurse finds this funny and laughs at them.
The movie has a happy ending, the protagonist manages to free his son.