After brushing the teeth and tongue, you could have him try using anti-septic and warm salt-water solutions. For the salt water solution, one can use both baking soda and table salt and mix it with the water.
After brushing, you first use the salt water solution and rinse the mouth thoroughly for about a minute or so. Then use the anti-septic. You can do that about three times, alternating between the two. Be careful to not do it too much, so that it does not irritate the gums. So, one has to use their own judgement, on how much you would have to do this. That would be an inexpensive method to try. It may or may not work, but he could give it a try and see....
I've heard that bad breath can be the result of other medical or dental problems. So, there are those things to consider too, if these methods of cleaning the teeth, gums, tongue and mouth do not work. In that case, he may want to see a medical professional.
I hope this common sense inexpensive method works, in the meantime.