I cooked a piece of pork shoulder and i put it at 250 celcius for 30 min and 130 celcius for 1 hours. Now the internal temperature is well above the 71 celcius mark and the meat looks tough. If i put the oven at a lower temperature can the meat soften up and make it tender?
Your pork will not become tender for quite a while. If you plan to shred it, you'll want to aim for about 205 degrees F / 96 degrees C.
You don't have to go that high if your goal is to slice.
Either way, a cut like pork shoulder is best cooked low and slow to allow all of the marbled fat to break down. It's an incredibly resilient cut of meat, but it requires patience.
While it is technically "done" as in safe to eat, it's not reached its culinary potential.
The toughness is caused by shrinking the connective tissue in the muscle with heat. There is nothing you can do, post cooking, to make it tender. About the only thing you can do is shred it.