You may want to try this...rather than feeding him while you're in the shower, stuff a few kongs with his food. Pack it in really well and add peanut butter to cover the large hole. This will keep your pup busy and his mind active. If he's too quick to get out the kibble, mix some peanut butter into it, stuff the Kong, and freeze for a few hours before giving it to him. You want to start this off while you're at home, on weekends for example. Leave the room for several minutes while
he's really into it gradually increasing the time. It's a good idea for him to practice quiet time and associate good experiences with his crate while you're at home as well as while you're away.
Not all separation anxiety expresses itself with howling and barking. With milder forms, it can just be the dog salivates frequently, breathes rapidly/pants, is physically agitated (turns in circles in his crate/is unable to settle), chews his paws or nails, urinates and/or defacates, etc., etc.. His peeing due to anxiety is out of his control. The only thing you can do is to teach him to become more independant and secure...with patience, practice, consistence, and praise at the right time.
You may find this article helpful...
http://www.sfspca.org/behavior/dog_l...ep_anxiety.pdf
Hiring a responsible, reputable dog-walker might be to your best advantage as well.