From my limited experience with Jacks, one thing I have noticed is that things change when they get to around the 5 or 6 inch mark. What was once a sociable fish starts getting agressive and moody, and fights can break out over nothing, especially between fish that had never had a conflict with each other before. Fortunately you're lucky enough to have a big enough tank for each fish to stay out of the way of the other, and if one of your Jack's teeth are getting like that big, then I'd stay out of his way too - lol.
Once his teeth have settled down he may get his appetite back, but you could try them on salad prawns for a change (though they get a lot of variety as it is).
Also - as C man says - are you sure you've got one of each gender ? If there's two males in there then things are going to get very messy very quickly. One thing that can happen with two males in the same tank is that the sub male will loose his colouring so as not to 'challenge' the alpha male. Is the other Jack showing stronger colours than before ?