My information is all second-hand, based on spending loads of time in the home ed community over the years, but I may be able to point you in the right direction for some things. The go-to resource for newbies learning about doing exams while being home educated is the Home Ed Exams wiki: https://he-exams.fandom.com/wiki/A-levels Don't be put off by the clunky interface. From there you will find links to some good Facebook groups. (Facebook is currently the main way home ed parents communicate.)
HE kids who are doing A-levels nearly always do them at college or sixth form, because they need a good deal more support with learning the material than with (I)GCSEs. It follows that most home ed discussions about exams are all about GCSEs. Be sure to chuck the phrase "A-levels" in at the beginning of every conversation in order to make sure you're getting the right advice.
I only know a few kids who've done A-levels entirely under their own steam, and they were exceptionally bright. You'll probably need tutors. People sometimes get burned by working with tutors whose only experience lies in supporting for young people who are being educated at college, in which case the tutoring is supplementary. It's a different kettle of fish where the tutor is the young person's only source of teaching, information, and help. For example, tutors who don't work with HE kids may not understand the marking scheme inside and out, may not be familiar with the specific syllabus you're following, and may not have connections with exam centres. Make sure you ask all the right questions when engaging a tutor!
By the way, if your son wants a second opinion on whether he's on track for the results he wants, you can use a service such as https://www.markmypapers.com/ to mark his mocks. They use professionals who work marking papers for the exam boards, so they know what they're doing. There are other such companies too, but this was the one we used for my child's GCSE. I was really happy with the service. I think we paid about £30.
You can do science A-levels without the practical endorsement, but probably those won't be accepted if you want to read a science subject at uni, which I guess your son does?
If your son decides not to do German A-level after all, he should check how the universities want him to demonstrate his proficiency in German, since he won't have done the Abitur there. My German son-in-law did something roughly similar to what your son has in mind, and proving his German language skills to the universities was an expensive last-minute hassle! (My SIL was applying to uni from abroad, however, and I imagine it will be more straightforward for your son to sit the relevant exam locally.)
Will he have his A-level results in time to apply for autumn term, or is there some other plan?
I'm not familiar with the law on compulsory schooling in Germany. Will your son have finished compulsory education age by the time he begins his A-level studies? If not, will his studies meet the legal requirements?