Separation Anxiety

Question
Hello,
I have a 4 year old toy poodle who suffers from separation anxiety. Now that I think of it, it started when he was a pup and has progressed. When we got him, he was the only dog we had so he was comfortable being left alone but he was hardly ever alone since there was always someone in the house. But even as a young puppy, he never liked going to other houses. He'd cry until he was back home. He's always been very anxious.
He's not necessarily attached to any one person in my house. I live with my siblings and parents. He just doesn't like to be left alone. As time went on, we got other dogs, and that has helped his anxiety when we're gone. Sometimes, however, he still cries and howls.
I am planning on moving out and bringing him along. He is going to have another dog companion but I don't think it'll help him. I know this is going to cause him great stress because we're going to a foreign place. I know he will be fine as long as we're there with him but he's going to panic when my partner and I leave for work.
My parents have a vacation condo and he's been there quite a few times and every time we leave him alone he cries and howls until we come back even when the other dogs are with him as well. We've tried leaving treats for him to work on while we're gone but he gets so anxious he won't touch them.
He doesn't do anything destructive to the house, he just cries. As soon as he sees us getting ready to leave he starts whimpering and shaking. I expect that he will act the same way when we leave to the new place.
He doesn't need to be by our sides at all times. He just needs to know that we're there. He's very comfortable being left alone in a room while you're somewhere else in the house. And, for the most part, his anxiety at home isn't as bad as when he goes to another place.
How can I help him overcome this? I'd like to make the move run as smoothly as possible for him. Any information regarding the matter is greatly appreciated.
Thank you.

Answer
Here's the hard part. With him being already 4 years and has progressed since he was a puppy, this is going to take time. First answer some questions; When you leave or come back home, do you give him lots of attention and spoiling? How long is left alone at one time? Do you crate him or leave him free in the house? How soon are you moving?

We need him to realize that it's okay for him to be left alone. Things that make that happen are when you leave or come back home, be calm and natural. I wouldn't even pet him right away, especially if he gets worked up. My dog always get excited when I come home, even though I've never antagonized that issue, but to this day (and she's 8 years old now) I have to wait and not even pay attention to her until she calms down. If he knows tricks, you that as a distraction as well as a reward. But the last thing you want to do is make an anxious dog more anxious. So only calm attention. No high-pitched voices or things like that.

As far as going to other places, it's good to have him adjust to new places and situations. But don't baby it. Make sure you stay relaxed and he'll sense that realize there's nothing to freak out about.

What you can do to test this and gets started is leave and either don't say anything or a quick pet and walk out. Just stay out for a few seconds then come back in, again with little or no attention. Repeat this and stay out for longer each time. Remember to reward when the behavior is what you want. And keep calm attention.

When he's at another place, are you there as well?

Feel free to give me some feedback and answer these questions for me and we'll get him more relaxed.