Hi Oognip
No doubt someone from Baselland will come along and give you more precise info just for your canton, but here's how it worked for me in Vaud, just to give you a rough idea in the meantime.
My daughter is in 1st yr enfantine (KG in the French-speaking cantons). She goes Mon, Tues, Thurs, Fri mornings 8.30-11.30am, then back Tues and Thurs afternoons 2-3.30pm. Our enfantine is part of the local school, and children with working parents can go to the lunchtime/afterschool canteen (except Weds pm, when the canteen is closed - not sure what the KGers do on Wed am, either).
Our cut-off school year date here is June 30th, so any children who are aged 4 at June 30th can go to enfantine at the end of August. So some might be 4 and a couple of months, whereas others may have actually turned 5 during July/ August by the time they start at the end of August. Different cantons have slightly different cut-offs, but it sounds like your son would be in the older half of the class, which is usually helpful for boys, who are sometimes slower developers of key skills like sitting still and concentrating!
My daughter had absolutely no French when she started, and now apparently understands most, if not all, of what's going on. Remember that the language for this age is (a) actually very limited in terms of vocab, and (b) very context-based, with lots of clues from pointing and demonstrating how to do a move in gym, etc. So it's not as daunting as adults may think.
My daughter has two teachers on a job-share; one has basic English, the other good English. They are helping her learn French, giving her sentences to model, using some of her well-known books from home to translate 'on the fly' into French for the class story (so she already knows the story/pictures); she also has a little group class once a week with a French tutor, although this is more unusual (and not really strictly necessary, as the teachers are so proactive).
In my daughter's class, she was the only non-French speaker (the others had done pre-school), but at least a third of the class has a separate home language (Spanish, Ukranian, Albanian, etc). The KGs are completely used to kids with no or shaky French (or German, in your case) and the teachers are experienced at coaxing them through the early days with lots of pointing and repetition (and usually some basic English anyway).
Perhaps there are some local language activities he could attend in the meantime - story time at the local library, a music session, something like that, that would go a long way towards tuning his ear to the sounds so it's not all so strange on Day One. Or see if you can find a local teenager to come and play once a week - most will have enough English to have very basic conversations, and could play with Lego to help him learn colours, numbers, how to ask for things, etc. Or perhaps mix-and-match his current daycare so he does a couple of days at a Swiss-German speaking nursery, or a home-based childminder.
But don't worry if these things are not available to you. Really, he'll be fine after the first few weeks, and he'll have two years before 'proper' school and anything academic. And the annoying little tykes develop faultless accents within minutes...
kodokan