Floor plan of a single-family house with approximately 150 sqm on a rear lot

First, I would give the advice to also record this in the land register – with all rights and obligations. Otherwise, a sale by you or him would have the effect that these "agreements" no longer apply and there is huge potential for trouble.

If joint use is intended anyway, why not completely – that is also with access to his garage from the north.
Can you roughly mark his house on the site plan?

But why should the children's rooms face west? A new house is being built relatively close there. Both children's rooms facing south (e.g. with a dormer) would provide a much nicer view. The bungalow to the south also looks quite new, the property is also divided, but used together, and with the fairly new bungalow I do not expect that to change soon. Access for a rear tenant would also be difficult there. The nice and "private" side of your property will be the south and east (there are trees in front of the railway embankment there).
My concept would therefore be children's rooms facing south, bedrooms probably northwest (actually rather east, but how loud/heavily used is the railway line?).

a bit more west orientation of the garden.

His house is visible on the site plan with the parcel number 190/25. He is renovating the existing building and is not constructing new (currently in progress).
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The railway line is only used once a day on weekdays (garbage to and from the transshipment station at the port).

That sounds understandable. From our point of view, however, there is no such necessity; we are planning with the 3m driveway that belongs to us. The fact that the neighbor’s driveway is not enough for him and he wants to use ours as well is a concession on our part.