If he keeps it as a small-scale bedroom business and doesn't invest much money in it, then let him do it. The beauty of selling MP3s is no capital outlay, so dissuade him from producing vinyl or CDs. If he's selling the music he created himself as MP3s via online music stores, then there is very little financial risk. If he starts signing other artists' music, then he needs to be more careful.
So other than that, the process is the same as setting up any business... register with the Finanzamt, get a tax number, etc. Lots of general threads on Toytown already about setting up businesses.
He's not going to get rich, but it keep him off the streets and maybe earn him some pocket money.
Wise words to my ears. But what if he already has registered with Finanzamt as musician (with a related tax number)?