Against jax, Jax’s Ultimate t will give him increased defensive stats which will make ganking a little easier for him as he will be tankier and deal more damage. If you meet him in a 1v1, avoid fighting him when his Ultimate t is up. 

Once Jax gets his first item, he becomes much harder to duel unless you have items that match his damage. Don't over commit to fighting him without a backup plan. 

As the game goes along, Jax will get stronger and stronger. Keep an eye on him at all times as he will look to split push and take objectives quickly. Against jax, If you encounter him in a 1v1, avoid fighting him when his Ultimate t is in place. Once Jax gets his first element, it becomes much more difficult duel unless you have elements that match his damage. Do not commit yourself to fighting him without a backup plan. As the game unfolds, Jax will become stronger and stronger. Keep an eye on him at any time he will seek to divide the thrust and take goals quickly.