Dog with Major separation anxiety! URGENT?

I adopted a six month old terrier mix last week and his seperation anxiety has been getting really bad. I can not even leave the kitchen( that is where he stays during the day) without him crying and howling and scratching at the door. It doesnt matter who but he needs to be with someone at all times. I have tried giving him this over the counter dog relaxation fluids for anxiety but it doesn't seem to be helping much. Its really terrible, i cant leave him downstairs without him whining. I give him lots of exercise- i walk him 3-4 times a day for 15 minutes each and do some obedience training too. Me and my mom went out today and we came home and the kitchen door was chewed up. I put some vinegar on it but i still hear him scratching at it from upstairs. He has a crate which he sleeps in at night. Should i lockhim in the crate when im not with him? he has lots of toys. I tried giving him peanut er kongs and boned before i leave but he still cried and chews. Iv tried everything. tommorow when i go to school and my mom is at home i will put him in the crate i guess- unless you think thats a bad idea- with a toy and treat and maybe il try puting a piece of clothing in that has my scent in too. and il leave the radio on for him to hear. None of this really seems to be working. advice? tips? Dont say get another dog to keep him company, because although id love too, it was a big enough stretch getting my mom to get a dog in the first place i can imagine asking her for a second. I really love my dog and i hate to see him crying and anxious. please help!