Right, am back.
First off. Get him to the vet for a check-up. Any sudden change in behaviour could potentially be down to ill health or pain, so make sure you have eliminated that possibility first.
Next, there are two books that might help:
Don't Leave Me by Nicole Wilde is the better one of the two, as it looks at the whole picture and you go through a process to help you properly diagnose the problem. Not all howling is down to SA. (The link is to the Kindle edition, but if you want a paper copy you can find them by googling - and possibly on Ebay.)
The other one is /I'll Be Home Soon by Patricia McConnell which is still good for helping you understand and assess the situation, but imo isn't as good.
It's most likely that he has just got used to you being around all the time and has unlearned how to settle himself when alone. If you watch him before you go out, he is likely to start getting a bit anxious as you start your leaving routine. So, say, you always do your hair in the hall mirror, put on your shoes, pick up your handbag and then keys etc, he is likely to be picking up on those actions as signals that you are about to leave him and his anxiety will be building before you've even left the house.
You can desensitise him to these things by doing all your usual exit routines regularly throughout the day, but not going out. So for example, pick up your car keys, then sit down and have a coffee, pick up your handbag and watch some tv, put on your shoes (perhaps not coat in this heat) and read a book. Doing this randomly several times a day will gradually desensitise him to each action as a potential signal that he's about to be left alone.
You also need to get him used to being on his own, rather than following you around the house all day. As others have said, a kong is really useful for this. Stuff it with super gorgeous dog treat type stuff (you can google for loads of recipe ideas) give him the kong and then leave without saying a word. Start off by literally just going to the door of the room you are leaving him in and waiting a second, then touch the door handle, then step into the doorway, then step through the doorway, then close the door for a second, then stand on the other side for a few seconds and gradually build up the time you are out of sight until he is happily noshing on his treat for a good 10 minutes or so while you are in a different room.
Every time you return, even when it's only a few seconds at first, immediately remove the kong, as we want him to connect being allowed to eat it with your absence.
The above process counter conditions your exit from something bad that he needs to be anxious about, to you leaving the room meaning something great - ie a tasty treat - will appear.
Start leaving him on his own with his kong for short periods several times a day, gradually building up the length of time he's left. To start with it may only be in increments of seconds or half minutes, but once you get up to a few minutes you can extend the gaps more quickly.
We found it was a good idea to do a longer session in the evening when he was already tired. We could put him in his bed with a treat and he'd was happy to sit in the kitchen with the door shut while we watched tv in the next room.
Once he starts to relax and is happy to be left while you are in. You need to start desensitse him to the sounds of you actually leaving the house. So, same process again, but start to include opening and closing the front door, stepping outside then straight back in etc, gradually building it up as you have done before. Once you can go out of the door and wait a few minutes, you may need to desensitise him to the sound of your car door and engine as well.
I recommend either filming him on a camcorder or webcam to see what he's up to - or if not - at least do what we do and buy a cheap dictaphone so you can hear exactly when he starts making a noise and if anything specific might be triggering it. It also helps you to see there is progress, even if it's only a few seconds at a time.
Finally, we have our boy on Dorwest Herbs, Scullcap and Valerian tablets, which are herbal tablets to reduce canine anxiety and are often used around bonfire night for dogs that are afraid of fireworks. We also have their Valerian Compound (link at the bottom of the tablets page) for times when we can't avoid being gone slightly longer than he can cope with. Some people give their dogs Rescue Remedy in their water bowls or just prior to leaving instead and lots of people have found DAP diffusers and/or collars helpful to calm their anxious dogs.
Where does he sleep? Does he or did he ever have a crate? If so he might like the security of having a crate to be in while you are out - doesn't work for all dogs, but often dogs that were happily crated as pups can find it reassuring to have one as their place of safety.
The other thing to consider is not to leave the dog for longer than his current tolerance level, so if he can only manage 10 minutes, try not to leave him for any longer, unless you absolutely have to. You do need to recruit an army of dog sitters and walkers, but it sounds like that won't be a problem for you and it's really worth it, as the barking and howling is self-soothing and rewarding. Barkng/howling releases endorphins that reward the dog for doing it and that's why it becomes such a viscious cycle. If you can do everything you can to prevent him from howling while you do the SA programme it will really help his progress. I do understand how hard this is though, not least of all because I was virtually trapped in the house, other than 10/15 minute school runs for the best part of 6 week, but it was worth it, because persistence and consistency has paid off and we are now seeing great progress that seems to be accelerating.
Sorry it's such a long post. It is a lot of work, but is doable if you are at home much of the time.
Hope some of that is helpful to you.
Feel free to pm me if you think I can help any further.
Good luck.