Well, I hate to even be considering some possibilities, but will speculate this at least for the sake of discussion:
If he was in fact a gummint spy at the time of his bizarre VVAW activities, he could have been creating bona fides which would make him appear as a 'friendly' of reknown to the commies when he went to Paris and elsewhere.
One thing for sure. If ALL be known, Kerry will be found to be a man of mystery who took part in a disastrous intel operation or the quintessential and consummate fraud . . . and as you infer, a favored protege of Kennedy.
If the former, it was not without dire consequence, but my gut feeling says go with the latter.
If the former were true and he was a FBI/DOD mole, it would distroy him in the Democrat party and with Socialists for sure.
However, for the latter, his dad was very connected with the Socialist movement in the USA.
Freeper Fedora did a nice post the other day re: his family and communism in the USA.