Jax was the runt of the litter but born even smaller then most runts were, the litter was a large one with 5 pups so that would be part of it. Akira was always protective of her pups but even more so of little Jax as even his siblings could be very rough with him even without meaning to. His sisters were destined to live in other packs as per pack rules only outside females may live in the pack, any females in the pack must leave and find their own mates outside pack territory. Males however are free to choose to live in the pack or leave but no male may breed with the females except for the Alpha, therefore ensuring total loyalty to the alpha as most males are the sons of the Alpha.
Jax idolizes his oldest brother Jett and quite often you can see him tagging along following at his brothers heels as he comes back from a hunt. Jax doesn't mind playing with the younger pack pups but he'd much rather be out practicing to fight and hunt with his brother, even if his mother tries her hardest to stop them both.