My left knee gives out some times when walking down hill. He doesn't really hurt but sometimes pops. Should I be concerned?

Slight concerned. You seem concerned enough that you posed the question. Because of the popping and giving out you should have your knee evaluated by a sport's doc or ortho surgeon. From you symptoms I would question some patella subluxation and quad weakness. B I cannot rule out some other mechanical such as a torn meniscus. Your symptoms may be resolvable with some physical therapy.
Vague. It might be weak and need some strengthening. Both knees, actually. I would see an orthopedic Dr. And be checked just to be sure.
Yes. Every time your knee give out you have to be concerned, also when your knee pop you shoud be concern. Please check it out with orthopedic surgeon. Be carefull. Thank.