What the hell?
Not that it matters -- big sideburns of '70s style certainly existed on some pop-culture figures before the '70s (and, no, you don't even have to go as far back as the 18th Century, or further, when they were all the rage) -- but from what I know of Engelbert his sideburns in 1966-67 (as seen on the cover of the "Release Me" LP, complete with rather startling bare patch mid-sideburn) were about what Elvis sported circa late '67 and early '68, basically not much bigger than Elvis' '50s versions. No high collars visible, either.
Of course, there's that '74 Tahoe concert (the 27th, I think) in which Elvis introduces Billy Eckstein and credits him for the high collars but, again, the high-collar thing was most likely largely an Elvis innovation based on his request of Bill Belew for the black leather suit in '68. And again, of course, we can go much further back to see that this sort of collar was hardly Bill's creation; it's not called 'Napoleonic' for nothing. The evolution of the jumpsuit (another item many take spurious credit for) is apparent from the karate origins of Elvis' two-piece 1969 stagewear and the preference for high collar follows Elvis' longstanding '50s (and sometimes '60s) habit of popping his collar upright on regular shirts.
In other words, I think the Humpster -- cool dude though he may be -- is off base with any claim that Elvis borrowed either hair or costume from him.