You can't.

And the most amazing thing here is that you can't get backward unless you buy a TTL/USB adaptator to plug in the docking port in order to reflash your boot2 (and then your OS).

OS 3.6 adds an anti-downgrade protection that removes your freedom to do whatever you want with the machine you just spent $150 on.

Do I have any other option for that to be possible

http://www.shopping.hp.com/en_US/home-office/-/products/Accessories/Calculators/NW280AA?HP-Prime-Graphing-Calculator

Else, what Levak said is the best solution, else you can re-sell your Nspire then buy an older one that still runs OS 3.1 or lower.

Not in order to have CAS, but rather to have a calc where the company dictates what we can do with it. That's an option, but it is more if he can't stand the restrictions that TI have put in the Nspire. I personally know that if I bought an Nspire and couldn't do anything I want with it, I would either get a refund from the store (which often works over here) or re-sell the calc then get a different or older model. Of course if he follows your suggestion then he can re-gain the ability to use nLaunchy, though. The $10 price tag for an adaptor depends of where he lives, though, because in Canada, shipping is sometimes twice the original item price, but it should hopefully be less money than re-selling the calc to buy another.