In Germany titles are part of the name, rather than a title of stand. So, he is legally adopted and is allowed the word "Prinz" as part of his name but he will never have more power or benefit than any other German citizen. Also the real family, carrying the name, doesn't recognise them as one of their one, since the lack the blood line.
Titles got abolished at the start of the 20th century in Germany. But as I said, you can still carry them as a part of your name.
Frederic dumbass von Anhalt told (dunno what is the truth with him, tho) that he was in negotiation with a UK title holder to get a "real" title himself.
We won't take them seriously, but their grandiose delusion and their tendency to invade the US, because no one in Europe takes them seriously, gives them some feeling that they are actually carrying a real title.
Useless fun fact: for some reason, a lot of these guys, with bought "titles"/names, are actually brothel, dance club owners