I'm afraid there is no tactful way :(
I would discreetly take the co-worker aside and say to him that you (don't mention the others cos the more that he thinks have noticed, the more humiliated he will be). Mention to him that you have visited some sites regarding ostomy surgery (this might let him know that you're not being mean, but are interested in what he's going through) and have read that there are deodorisers available if needed.
To be honest, there really should be no odour. The bags are odourproof. The only time there should be odour is, of course, when the person goes to the loo (but then even 'bowel intact' people have a problem with odour while in the loo) or if the person has developed a leak in their bag. This person may not realise that he is causing a problem and may be greatful to you for pointing it out.
You could even suggest that there are special ostomy deodorisers available but 3% hydrogen peroxide is one of the cheapest (and most effective in my opinion).
Good luck. Do try to be sensitive but given the nature of the problem, it's going to be hard to tell him I know.
I have had an ileostomy for 32 years now due to UC.
Moderator of the Ostomy Forum
I'm not a complete idiot - some parts of me are missing!