I'd put him in there right now without problem. I just got myself a 3 month old panther and he's living full time in the free range below, and that's something like 5'w x 2'd x 8't:
I've been doing this for the last 2 weeks I've had him and he's been doing fantastic in the natural sunlight. I hand feed him about half his food for the day and let the remaining half go on the screen and he picks them off as we wants, and even with all this space he manages to catch absolutely all of them. So your guy in a 2x2x4 cage will do just fine. If they can manage to survive in the great expanses of the outdoors, they can definitely handle hunting for crickets in a 2x4 cage
If you're uncomfortable with that idea, they can be moved to that size at about 6 months. 6-8 is the norm, I think.