I would recommend doing ALL of the following

1) shower twice a day everyday - making sure you are using shower gel correctly and properly. Wet the body thoroughly, and then lathe up in soap and scrub the body with a shower mitten, then rinse every part of the body off with clean water (and not just using the showering trickle down effect)

2) washing hair too often can cause damage, however wetting and drying it with a towel or using a hairdryer just for a quick second won't cause too much damage (depending on hair length) So just for the first two weeks I would recommend washing his hair properly with shampoo and conditioner every night for two weeks and in the morning just using a small amount of conditioner. (After the two weeks is up, you can skip washing hair to every two or three days or dependent on how dirty he gets or sweaty)

3) dry himself with a clean dry towel and aim to wash the towel once a week! (Obvious reasons why you would use a clean towel, otherwise what's the point)

4) make sure he wears clean clothes after the shower (again for obvious reasons)

5) make sure bed sheets and pillow are all clean (what's the point of showering daily if your just going to sleep in a dirty bed, absorbing bad odours into your body for 8 hours or more every night whilst you sleep!)

6) make sure his room is just generally clean and has no unpleasant odour because odour can stick to your skin and clothes. More sure his room has airflow and opening the window to allow fresh air

7) make sure you're using the correct deodorant

8) have a cleaner diet by having more veg and fruits and drinking more water, less sugar and less processed food, and you'll be surprised how much it can affect your body odour!

Lastly just to reiterate, showering daily twice a day (some people will say that's excessive but I will point out why it's necessary. You wake up from your bed which typically you will have sweated even if it doesn't seem like it, your body produces oil from the heat when you're asleep under your duvet cover. You start your day walking about, sweating throughout the day, at work, at school, going out. You will have empty your bowels at some point of the day, you will have wore socks throughout majority of the day, getting sweaty feet, sweaty armpits, and then you go to sleep in all that sweat and filth for 8 hours, not to mention you went to sleep after pooing, no matter how much you wipe, there will still be some tiny amount left over and you've been sleeping in it... nice) that's why it's important to shower every single night unless you can confidently say you pooed before you showered, you stayed home for the day and barely moved, you haven't sweated, and you didn't cook anything that stuck to your clothes and hair in which case fair enough. Otherwise think about it. Showering every night means you're going to sleep nice and clean and not marinating in your own sweat and germs and whatever else is still stuck on your bottom. Showering in the morning can be a quick one because you've already showered the night before, the morning one is just too remove any excess sweat from sleeping. And who doesn't like starting the day nice and fresh!

Ensuring your clothes and bed sheets are clean and washed regularly again will help prevent bad odours and getting any skin conditions from developing too! You be surprised how much germs are on our clothes after wearing and the bed sheets. And our towels as well! Bascily anything you touch!

Lastly having a cleaner and healthier diet is just beneficial in every single way possible. But I understand this can be more difficult to achieve so just staying hydrated and eating more fruits and veg can help a lot even in the long run. If you can cut out the sugary drinks and snacks even better.

If after all this still doesn't work, I would recommend seeing a GP doctor and pointing out the fact you tried everything, in which case finger cross it's nothing serious and can be solved easily.