I agree with TxVicki.
Has the pup had all of his shots? you are suposed to wait atleast a week after their last lot of shots before you take them out for a walk.
That and the fact he is on meds means there is no way this pup should be going out for walks or put on the ground in public places (or anywhere near another dog).
Please answer these questions
EDIT: Just read that the dog hasnt had all of his shots. He shouldn't be on the ground in a public place AT ALL, even if it is an area that dogs dont usually go in. You have no idea what nasties people have trampled in on their shoes. Please dont do this again!
And to answer your Q about seperation anxiety then yes they do have this for a little while. Poppy has it for a few weeks before she realised I was always coming back when I left the room.
The vanccine schedule I used for Poppy will be different to yours because it is my understanding that dogs in the US have more shots then here in the UK (because we dont have the rabies shot). Poppy had her first shot at 8 weeks then her second at 10 weeks, she went for her first walk 2 weeks later.