If you want to "get involved" and help your colleague, have a private chat with him (where nobody can hear the two of you talking) and provide him a solution for hos problem (please keep reading for such a solution).
If you just want to have the problem fixed, or if the colleague does not want to cooperate, "delegate" the talking to your boss.
Before university, I had a situation for a while, that my sweat had quite an unpleasant smell, in spite of bath / showers and in spite of always wearing clean clothes.
Just walking from home to school (about 20 min) generated enough sweat to turn me into a (don't-)wanna-be-skunk.
I did not know how to handle the things at that time, and the problem went away by itself.
However, now I have a solution. The bad smell does not come from inside the body, but from micro-organisms which live on the surface of the skin, and feed on the said sweat. Getting rid of the "buggers" fixes the problem.
Cheap, safe, easy solution: apply on the problem-area moist baking soda. Rub it on the skin gently. Let it work for about a minute. Rinse. Repeat daily for a few times, until the colony is gone. Repeat occasionally, as needed.
Alternatively, add 1/4 kg (1/2 pound) of baking soda in the bath water. It will work on the entire body. No special rinsing necessary.
The same baking soda will remove odors from shoes. I do it with my winter boots, and there is no stink in the boots, even in spite of (occasional) heavy sweating.
Caution: Baking soda can temporarily stain white dark clothes / socks. The stains will go away by a simple washing, just like any other dust.