Hello and welcome to the board. I'm sorry I didn't see your post before; sometimes this forum is a bit quiet but people do come along and there's lots of encouragement and support.
You have definitely spotted signs of drug use on your son's part, and you're right to be concerned. Drug use can completely derail a teen's life. Teens who are using pot are almost always using or at least experimenting with other drugs as well. Whatever your son eventually admits to doing, you may safely assume he has been doing much more.
It would help people on the board if you would provide a little more information about your son - previous behaviors, diagnoses or contacts with mental health professionals (if any), previous worrying symptoms that may have indicated substance abuse. If you make a signature line that outlines your family structure it will also help people respond to your posts.
As far as what to do now, I would suggest a thorough search of your son's room and bathroom, as well as your daughter's room, to locate any/all drugs and paraphernalia. If you smelled pot coming from his room he has it on the premises. Go through his room with a fine-toothed comb, open every container, unroll every sock in his drawer, etc. If you know he's creative, be creative in looking for hiding places. This is NOT an invasion of your son's privacy. It's a life/death issue. I take it he's still 17 for a little while, if he turns 18 this year; he's a minor and he lives in your house. You are responsible for his wellbeing and your daughter's wellbeing, and searching out illicit drugs is completely appropriate on your part.
I mentioned searching your daughter's room because unfortunately it's fairly common for teens to hide incriminating things in a sibling's room.
Once you find what there is to find, you will need to confront your son. But I hope you'll post a little more about your situation so that parents here can offer suggestions about that, and about treatment/rehab options, etc. You will find lots of support here.