For some workers, it's their sweaty armpits, and for others, it's their breath. What a difficult situation!
There are some general tips available in my article https://toughnickel.com/business/Somebody-Stinks-P... under the heading "How to Deal with a Smelly Coworker," but these may be less applicable to your situation since you're dealing more with a yucky body odor issue rather than a perfume and allergy problem. There could be cultural, religious, mental health, or age issues involved as well so tread carefully.
If you have a good relationship with your coworker and feel comfortable doing so, approach him directly but respectfully and explain the issue as nonjudgmentally as possible. Examples: "Frank, when the weather is hot, your deodorant stops working, and the odor is very distracting to me. Do you mind wearing an antiperspirant to reduce sweat and body odor?" or "Sam, when you exercise during lunch and don't shower before returning to the office, the sweaty odor is very distracting for the rest of the afternoon. Do you mind rinsing off before returning to work so I can concentrate on my job?"
If this is not a conversation that you feel comfortable having, then approach your manager immediately. This is what managers get paid the so-called big bucks for. Tell him or her these things:
1) Ask if the manager has noticed the smell. Describe what stinks (armpits, breath, feet, etc.) and just how bad it is.
2) exactly when the coworker stinks and how long this has been going on (every day? in the summers? since your coworker's father died? in the afternoons following exercise at lunch?)
3) who has noticed the body odor (coworkers? customers?)
4) what the impact has been (hard to focus on work, can't invite clients in without being embarrassed)
5) what solutions you've tried and how that's worked out for you, and
6) what you want the manager to do about it (propose a solution).
It's an awkward situation, but a good manager will handle it discreetly and professionally so that you can all concentrate on your work. A wimpy manager will not want to get involved. (It's his or her JOB to get involved.)