Hi Erin, our dog Chloe has been carried around in many diffrent types of bags and is very easy going. She has been smuggled onto air planes (pre 911) and into movie theaters and restaraunts so she took to the backpack with ease.
The main thing would be to find a pack that is comfortable for your dog so he can sit up comfortably and can have his head out. The pack we use is an old Edie Bauer pack with an upper and lower compartment that is kind of pear shaped and we put her in the lower compartment and with nothing in the top compartment.
I put her in before putting the pack on when I am by myself but it works better if I put the pack on then have someone else put her in. All I have to do to get her out is to sit down on the ground and she can hop out unasisted.
Let us know how it works out, and good luck.