Make a mixture of 1 cup of bleach with 1 gallon of water (or any other 'relative amount' therein.) You can use a spray bottle or a brush with a mixture in the bucket. In either case, apply the mix to the mold and mildew. If after a few minutes you see a color change, you know it's mold and mildew. If not, you know it's just dirt.
Then, clean up the mess any which way you can. If you want, you can make the mixture stronger by adding a little more bleach or by taking away some part of the water. Just be sure you have ventilation if you make a strong mix. Bleach isn't good to inhale at all.
Your son's next step is to see if he can do anything to mitigate the water-in-the-basement problem in the first place. Good luck! My best to him and hope this helps.