Hi there, I'm looking for some advice on how to help my dog with separation anxiety.

He's a 5 month old Lhasa Apso x Bichon and in every other aspect he is wonderful and such a lovely dog.
We've had him since 8 weeks old and have attended puppy training classes which he has been a star in but he just won't be left alone.
Neither of my parents work, so they are with him during the day whilst I work so he never really needs to be alone, but this week my parents have been on holiday so I have had the week off work to look after him.
I can't even shower without him barking his head off. He follows me everywhere.
I have had to leave him alone every morning whilst I attend to my horses, dogs aren't allowed up. He has only been left alone for an hour tops but has destroyed the place.
I tried leaving him in the conservatory, with the radio on, his toys and a kong ball filled with treats and came back to all the wine glasses on the dining table smashed all over the floor and poo everywhere. I tried leaving him in his crate which is in the conservatory, but again there was poo everywhere in it.
I tried the kitchen with the radio and toys and a bed and came back to poo and wee everywhere. I tried him in the living room with the TV on but again the same and have this morning come back to a letter from my neighbours saying they are going to report me if it happens again because he does not stop barking and it's cruel and that I should take him with me everywhere.
He isn't left on his own long at all but the odd time he needs to like if my parents go food shopping or something.
Does anyone have any suggestion on what I can do to help him stop stressing, barking and pooing everywhere.

I highly recommend kennel training.

Dogs like to have a den. In the wild dogs spend their down time in a den. It's their safe place.

The kennel should have solid walls, a lookout (wire door), and be big enough for the dog to stand, turn, and lie down. No bigger than that.

The good news is that your dog is still young, and should adapt to a crate very well. Start by crating at night, and then during the times when you need to be gone.

I have two dogs. One is crate trained, the other isn't. I have an old crate in my garage/workshop, and they both love that crate. There's no door on it, they're free to go in or out as they please, and they both prefer to be in. It's their den.

I highly suggest the crate. A room is just to big. We humans want the space, but when a dog is left alone, they want a den, a smaller cozy somewhat confined (by human standards) space. It makes them feel safe while you're gone.