Earlier quoted context omitted.
Ask your brother to stop the intake of sugar completely for 3 month (including all the hidden sugar in any processed food). I know this from myself and couple people which had acne that if you stop the intake of artificial sugar long enough the acne will mostly vanish. Even today whenever I eat high amounts of sugar I know for sure what will happen. I think this is a good example of misuse of antibiotics.
There are many good reasons for cutting down on sugar, but before we get enthusiastic, it is worth recognising that there is no established link between diet and acne. See the 'common myths' section. http://www.nhs.uk/Conditions/Acne/Pages/Causes.aspx
Also it's generally not useful to provide links like this. I can create hundreds of "sources" for you which say its myth. I can also do the opposite. If you want to link something please link a full blown study with hundreds or thousands of participants over at least half a year. Then we can talk.