I think the easiest way of doing this would be to study in the UK but spend a year abroad on the DAAD scheme or go to Germany for postgraduate studies (masters which as someone said above is the Point at which most German companies would consider it a completed degree). They have introduced BAs but they are not yet widely accepted as a full degree. So a degree = 5 years in total to MA Level provided you don't mess about and stay on the ball. Can also take longer. Once he is at a UK university, I am sure someone in his department would be able to recommend a relevant place to apply to in Germany. I am not sure how far he would have to be in his studies in order to be eligible, you'd need to check the DAAD website. I think if he goes via DAAD, they help with accommodation.
If he is going to go it alone and complete his entire degree in German in Germany, perhaps a smaller town is better, simply because German degree courses are not very structured and the smaller the set-up, perhaps the easier it is to find your way - also to get to know other students. Bigger cities like Berlin tend not to have a clear campus but buildings spread over town. Most of the TU Berlin's buildings are I think located in the same basic central area near Zoologischer Garten train station. At least AFAIK the mathematics and physics buildings are both at Ernst-Reuter-Platz. The FU Berlin is quite different and you can find yourself having to catch a bus or UBahn to get to your next course.
You do need to sort out for yourself what to do and when to do it. There is little to no guidance so he really needs to be a self-starter or you need to both give this some thought. You buy a Studentenhandbuch with a list of courses/lectures in it, figure out your time-table for the semester, check the boards for changes to the programme which may have come up after publication. There is also no real tutorial system although some courses are run by tutors and called tutorials, it is quite different to the UK, so ds would attend lectures and seminars he has chosen, collecting Scheine as he goes for the courses he has completed. Sometimes a paper is required for this, sometimes an exam is sat. With physics I presume a lot of it is exam-based but maybe they get Scheine too for practical work. These need to be picked up at the end of the semester, stored carefully and handed in together with a list of the courses/lectures he has attended proving he has the requisite number of hours before he can sit exams to complete his undergraduate degree.
If he is looking at the Technische Hochschule Aachen, they have a week long orientation set-up at the beginning of the first semester so that would be a help with finding his way around and knowing what he has to cover. Every university has a Studienberatung for foreign students (Studienberatung fuer auslaendische Studierende or similar).
Here's the page for Aachen for instance:
Beratung
with an overview of things he'll need to sort out - registration with the authorities (needs to be redone every time you move), health insurance, Studentenausweis (needed to get the reduced fare on public transport), accomodation, opening a bank account (can require some patience all this ).
The universities have Studentenwohnheime with rooms (communal kitchens and bathrooms) or 1 room apartments with kitchen niche and own bathroom. However, you must get in very early to have a hope of getting one of these. For accommodation help, you need to contact the Studentenwerk.