Training an Older Dog to Use Pee Pads?

I have a 4 year old Shih Tzu, not fixed. I thought he needed a companion so I adopted a 7.5 year old Shih Tzu who is fixed and potty trained and have had him for 2 weeks. He's had quite a few accidents!

I take him out every 6 - 8 hrs. My other dog is potty pad trained. Now the adopted dog will not take to using the pad. Please help. I am lost! How can I get him to stop using my rug as his pee pad.

By May