Generally around major veterans dates -- Memorial Day, Veterans Day, etc., Ancestry will allow free access to military records. If you know his unit on D-day there will be a record of that unit's movement through WWII. The unit's history. His military record, if available, will tell his role in that unit. Since he survived D-day and is not buried anywhere in Europe, then all of his records will be state side. If you don't understand or have trouble understanding all of the jargon in his discharge papers, take them to a local VFW posts and someone there should be able to translate for you. The first requirement is to obtain as much as possible of his military record. That is the focus.
Some of the questions you want to know is -- What was his unit? What was his unit role on D day. When did his unit come ashore, etc. Remember only about 1 in 10 were on the front line shooting. Could easily have been in a support capacity -- supply (quartermaster), communications, transportation, medical, grave registration, construction, etc.
I don't think a private guide in Normandy would be useful other in a very general way. Without a unit designation you know nothing and the guide is only guessing.
If it is a bronze oak leaf cluster that would mean a second awarding of the purple heart. So most likely wounded twice. The service record should indicate when and where.
PS. Just had some further thoughts. This might get very complicated for you. If he was wounded enough to be taking out of action for any period of time, most likely his unit would have moved on. So, on recovery, he probably would have been assigned to another unit in the area as a replacement. If severe enough he could have returned to England or another area for recovery and then reassign. You need that record.
Another thought -- Contact the WWII museum in New Orleans. They have some terrific resources and records. And if close, visit before you go.