desert "probably" from the "Volksdeutscher."
You cannot desert from the "Volksdeutsche". These were a group of Polish citizens that were considered ethnic Germans by the Nazis, and as such were conscripted to the German military. You need to read up on the whole concept to really understand what it was all about. There is a longish thread about this topic somewhere in PF's archives, if I recall correctly.
Did he see action while with the German Army?
Write to the so-called "Deutsche Dienststelle" in Berlin and provide all relevant information. At the moment, it takes more than a year for them to respond due to the large number of research requests. If they have a military record of your father on file, they will send it to you together with copies of personal documents they might have in their archive (such as an ID card that identified him as "volksdeutsch", his military ID card, and so on).
How long was he in the Volksdeutscher?
Depends on whom you ask. Many Poles consider(ed) these ethnic Germans (the Volksdeutsche) traitors, and they were treated as such. Since your father fought in the British Polish Free Army, things are most likely a bit different.