Yes, unfortunately true. The son of Robert Walker ("Strangers On A Train") and Jennifer Jones, he was always pretty much typecast because of his overly youthful appearance (he's basically playing a decade younger than he was in "Mind Over Mayhem" which he did at 34). His most famous role was the Star Trek episode "Charlie X".
One of his most untypical roles (I guess) is in something very famous. In the "commune" part of EASY RIDER, he plays a sort of guru figure making a very small speech.
One of the first things I think of (even though he has the closest thing to a "thankless" role in it) is a very underrated 1967 comedy called THE HAPPENING.
And he got to play the lead role in the comedy ENSIGN PULVER. It's considered a very weak sequel to MISTER ROBERTS, but I've always been fond of it.