If he really has no idea wht he wants to study push him to do the Stamford Test available on the UK's university central application service site - it's very good at identifying subjects that might interest him that are less obvious than favourite school subjects. This might give him a clearer set of ideas before he goes to the university advisors.
http://www.ucas.co.uk/stamford/index.html The deadlines etc vary a lot according to subject. If it's a subject with waiting lists (Numerus Clausus) this needs to be taken into consideration but the big one is if it's something like medicine, law or pyschology when he has to think about the deadlines for the ZVS, which allocates places centrally according to grades and location wishes. If he wants to study say Egyptology at a big city university, he can more or less walk in a couple of weeks before the start of term and register.
Once you've got the subject then comes the place. Some of the Fachochschule are great, better resourced that some of the universities, others are abysmal. Unfortunately there are no publically available quality indicators, so it's hard to tell. Don't rule them out though particularly if your son is more practice than theory oriented.
It's also worth having a look at least at the rankings for universities. The quality and standards vary pretty dramatically although no-one wants to admit it, and this does impact on job prospects. While no system is perfect the CHE rankings match up pretty much to what I know of the subject I teach.
http://www.che.de/cms/?getObject=2&getName...king&getLang=deSome things you might also think about, once you've got a shortlist of possibilities are:
How many students are completing their studies in the regular time? If almost none it's not a good sign.
Has the university already started the process of moving to the BA/MA model? They all have to do it and those who haven't started and are going to have to rush to meet the deadline are going to be possibly problematic to study in.
Look at the Fachschaft website if it exists - it's often a sort of alternative prospectus and honest about problems etc.
Finally, this website is aimed at foreigners wanting to study in Germany but explains a lot about how the system works for those new to it:
http://www.daad.de/deutschland/index.de.html