Thanks for the warm welcome and your valuable suggestions!!!
I carefully studied your methods and it looks like the combination of two polaroid dust and scratches passes (once on white and once on black) together with selective masking works best for this image. The thread of Caitlin gives another impressive example of using this filter. Unfortunatelly I didn't have it installed right now and couldn't check how it works for the whole image (the boy is just a part of a larger family photo).
Personally I decided to try a long route of cloning (thank you, Kraellin, for introduction). It seemed particularly appealing to me, as I bought a Wacom tablet just a few days ago and wanted to play with it.
See my result in attachment. The right side is treated with a healing brush in pattern mode, which I generated from a sample of the surface without blemishes. For the boy I used the sample mode.
It is very encouraging to experience such a great support from your side. Thanks again!