Thomas, first - and most important - you need to edit your question and remove your email address. Putting it on a public forum like this only invites spam (or worse) and any member can contact you by sending a private message, so it's not necessary here.
Based on what you said is displayed for your father's name, if you edit his profile I expect that you will see that his LNAB (Last Name At Birth) is DeLong but his CLN (Current Last Name) is de Long. His LNAB should be exactly what it was when he was born. If he later changed it then the new name should be in the CLN field. Based on what you said, it sounds like his name is showing correctly, in accordance with how WikiTree displays names. If DeLong was not his correct LNAB then you can change that if you're on the trusted list for his profile.