Make sure it is served in a way he can eat it (it might need to be chopped up), and make sure its fresh. Other than that, most torts will have a food or few that they just don't like (my tort gates collard greens for example). It won't cause him harm not to have cactus in his diet. Just make sure he has enough variety--I like to feed about 20 foods over the course of a few months for example.
my greek is almost a year old and it still wont eat cactus. i would hold off with it for a few months and reintroduce cactus when it is older. or crush it up and mix it with some mazuri. you can also crush it up and mix it with greens. dont be discouraged. your greek will probably love it when it gets older.
My sulcata hatchlings were eating it in their first two weeks. At first I minced it up super fine and mixed it in with other greens. It just turns to slimy mush when you chop it up really fine. Now I just put in a whole tender young pad and they are able to eat it just fine. They pull against the weight of the pad to chop up bite size pieces with their sharp little beaks.