Our LO is much younger than yours - she's only 19 months, but she absolutely ADORES jelly, although we have never had the obsession that you are talking about - maybe only 5 or 6 days in total where all she would eat is jelly.
What I do, when I think we are getting in a jelly rut, is to just not buy or make any more and then show DD that there is none in the cupboards and none in the fridge. Once she is satisfied that there is none, she will eat something else (even if only fromage frais). Actually, she hasn't had any jelly since Christmas, so we've not had that issue for a while.
I know you said he would starve himself if he didn't have any jelly, but maybe you should set yourself a target date for 'jelly removal operation' and deplete stocks and show your DS that they are going down and that once it is gone he can have any of the other yummy things you have to eat instead - I don't know whether or not that would work, but I am reckoning that DD's cousin (who is 3 and a half) would probably buy it....
