Pain & Stiffness

My Basset-Love named Barty is a 9 year old (will be 10 in September) male. He is usually happy, active, excitable, and social. I saw a major change in him on Saturday [yesterday] morning (literally overnight). I took him out to go to the bathroom, he was seemingly fine, but when it came time to go back upstairs (I live in an upper), he would not do the stairs. I thought he might have been upset because he knew that I was going to be leaving for work soon and trying to protest (he has some attitude). He did the first four steps to the landing without much difficulty, but then would not go up the stairs into our appartment, so I carried up him up the first set (six steps) to the next landing -- he yelped after I lifted him and decided to do the rest of the stairs (another six) on his own after much pleading, tears, and promises of treats. And with that yelp I knew he was in pain. I figured it might have something to do with his joints because of the weather (rainy & cold). I took him out when I got home from work and the same thing happened and he seemed to be wincing as he climbed the steps. He's also been slightly withdrawn, spending most of his time on his bed in the bedroom and didn't ask to play. The only time he has approached me has been around meal time (so he still has his appetite). The scenario has been the same today, except I noticed that his tail is down and I think that this is the thing that is upsetting me the most. I have never seen his tail down except for when he knows he's done something that he's not supposed to (I didn't notice his tail yesterday). And the weather thing is out since it was clear & sunny out today. I'm taking him to the vet tomorrow, but I just can't sleep knowing that he's in pain and I can't help him. I'm praying he'll magically be better in the morning. Any advice/suggestions/support is much appreciated.

Thanks.