I'm going to tell my co-worker he smells bad
Why do you want to tell him he smells bad?
I am not trying to offend him. Just want him to know the truth so he can fix it.
If you're right you should let your co-worker know that they smell bad. It's for their own good.
I just want to see how they react when they find out other people don't like the way they smell.
Well sometimes people need to know when they smell bad. It can be hard to always identify if your body odor is acting up or not, so you telling them this in a friendly way is the right thing to do!