caught my son smoking weed.?!

Talk to him about it.
let him know how you feel about it, the way it effects him, his family, school etc.
also though around sixteen is when teenagers become more expirimental, i tried weed
for the first and only time when i was sixteen. You can also buy home drug tests at most drug stores, start testing him every month ; ground him, take things away whatever you have to do to get your point across. Though he shouldn't be smoking weed, weed actually is the safest drug ; (not saying it's okay)

Confront him about it.

Don't physically hurt him or anything, if you do it can make you look worse of a parent and your child may disrespect you more. Confront him casually and ask him what it is and where he got it from so you can try to come to a conclusion to prevent him from getting that again.

Also you can try telling him the side effects of THC and smoking has on people, this can sometimes work as a scare tactic.

Esh, this is a tough subject, when I was younger I ended up talking to my dad about it, and he pretty much said "Yeah I guess weed isn't so bad if you have a job and you're going to college and what not, but if you aren't doing anything but highschool it makes you look and feel like kind of a slob, I'd just prefer to find you drinking, yeah its more unhealthy, but its more socially acceptable and I don't have to worry about you going to prison." Something along those lines, I pretty much felt like **** and I quit smoking weed, but I was also going into the Marines at the time, so I had more motivation to quit. If you just tell him to stop, he probably won't, my reason for believing he won't is he's either a dealer, because of the baggies you found, (well he's only a dealer in my eyes if those 5 baggies you found each had weed in it, and similar amounts in each one), if they were empty, it means he's been smoking for a while. So you have to take a more tactical approach to it, check his grades ect, nonchalantly bring guilt into it. Because its true, he should feel guilty, he's living under your household smoking pot, if he has a job I guess its not as big a deal, but he's still living in your house and needs to respect your home.
Also, try and find something to give him motivation, because my biggest issue when I was trying to quit smoking was motivation, I was so far in, I was smoking every day, it was bad, and if you smoke a lot it puts you in a pit, and you really need to find some way to motivate you enough to pull yourself out of that pit. It sucks but its worth it, I couldn't eat for like four weeks after I quit, because I felt like ****, supposedly weed doesn't have physical withdrawl effects but I beg to differ, nothing seriously dangerous but I still felt like **** for weeks. Albeit, I deserved it. I was also unable to sleep. And honestly, weed led to another thing, I ended up trying ecstacy because weed made me more curious over time.
Anyways, if he's dealing, that's a whole nother problem, because then he is in deep. And it isn't going to be easy to get him to quit. I did it by myself, without any therapists or support groups, so he can do it for sure.

Personal experience