Ahh, yes, now that you explain it, it does seem like a mental thing. Is he scared of people or random things? How is he to walk? other dogs? Whines a lot? Sounds like you have to build up his self esteem. If he has no confidence in himself and relies on you for everything, when you leave, he freaks out, regresses, and has bad behavior.. And the vet's is no fun to be for an extended stay, so he definitely had some flashbacks..
I'm assuming you don't keep him in a kennel when you go on vacation? I don't agree with kennelling in general, but there are some dogs that should never be kennelled for any reason- They're too sensitive to handle that kind of chaos. Even doggy day care can be too much. I would stick with family/friends/pet sitters to go to your house and take care of him when you go away. Maybe you should even have someone stop in and play with him while you're at work to break up his day and give him stimulation.
I would also exercise him alot, if you don't already. Like at least an hour hike a day. Excess energy can easily turn into nervous, anxious energy. If he's tired he's more likely to sleep while you're gone.
The hardest part is not coddling him if he's unsure about something. If a dog gets scared or wants to scramble onto your lap to be babied, or "velcro" to your leg whenever you go somewhere, the last thing you should do is comfort, pet, hug him, tell him its ok. Just ignore him and make him keep a space away from you, correct you if he protests by crying. Only praise him when you notice he's away from you AND not thinking about being next to you, (ie. when he forgets that he's not near you)-he'll come running, so send him away from you again and ignore him- have him come only when you call. If he wants to be with you 24/7, I know you really don't mind him there, but you have to let him know its ok to not follow you everywhere.
He'll eventually come around, be calmer, and more sure of himself, and his bad behavior will disappear. He also might be more independent and not have to be with you all the time, you'll think he doesn't love you as much, but it's not that at all. He's just a happier dog exploring his world, and it's better for him that way.
He's clear with health problems now. It's been over a week since he had IV fluids now, so it should be completely out of his system. We got him from a shelter, so I think maybe it's definitely mental because he had done this when we first got him, then took him to the vet, so not being with us definitely set in with his mental angiush. He's back to being more of a velcro-dog, then anything. The medication he is on, doesn't cause any of it. I called the vet and she said its just time to bring him back mentally stable. He's just over a year old, he turned 1 yr this past July. But I think leaving him in a kennel, by himself, for almost a week, he may have thought we just left him there and didn't want him anymore, like someone did to him when he was 2 months old.
Is he on any medications now and what are they? Some meds can cause increased urination or incontinence, as can age and underlying conditions. How long has it been since you brought him home from the overdose treatment? If he was given IV fluids to flush out the toxins, then he will most certainly have to "go" alot for a little while. If however he's still doing it and its been a while, then it's something else.. UTI, possibly? I would have health related problems ruled out before considering behavioral issues. Has he ever been away from home before? If this was his first trip away, then it may be anxiety, but I would check for health problems first.