"LEGITIMATE" LOL
I say take what you can get and enjoy it. If you find out after a few weeks or months or years that your Guro is a fraud at least you've learned what JKD is NOT.
My bet is the man know his business. There are MANY 3rd and 4th generation instructors out there TEACHING their version of "JKD" and they do it very well. Don't sell a man short for trying to make a living at it.
Generally speaking the few "associate" JFJKD instructors that I've met are gentlemen and scholars.
You can always supplement your routine training with seminars or chat with other students/instructors from other parts of the country or the world and compare notes... Odds are good that you'll be introduced to the same fundamentals. There is a structure or a "core" to the art. That's what makes a thing "JKD".