William was exactly the same when Louis arrived. William was 5 at the time and having another dog in the house came as a big shock to him. I made sure that he had places where he could get away from Louis to have some peace and quiet.
I also made time for William, partly by walking him as normal every day (leaving Louis in his pen with some food in a Kong) and also making a fuss of William, grooming him, playing with him etc. when Louis was asleep. When Louis was awake they were given treats together to help them get used to each other but if Will chose to walk away I let him.
When Louis was old enough for walks I gave them one short walk together each day, a long walk for William on his own each day and a short walk just with Louis. That helped Louis not to become too dependant on William and gave me time to train him and bond with him.
Louis is almost 5 now and he and William tolerate each other very well, but they're definitely not best friends because William is never going to let that happen. He's much more interested in people than other dogs and takes little notice of Louis.
So don't expect your two to become best friends. It very often happens and it's nice when it does, but you can't force the issue and as long as they learn to tolerate each other and aren't stressed by that they will be fine in the long run
