If he has been a regular boy until now you shouldn’t be too nervous, he is probably doing it to fit in with some of his friends, but that doesnt mean that your reaction isnt extremely important. If you would rather him not wear jeans, HE KNOWS THAT. ans he wants to see how much space you will give him.
In my opinion which is based on zero, its best not to react negatively but at the same time not to make believe you approve and at the same time not act as if your acting.
Hatzlocha, also check out priority-1.org for parenting tips