Congrats!!!!!
Hmmm...I'd maybe try another harness. Petsmart has a little area within the harnesses for super small ones, maybe try there. He may not be going right away right now both bc of stress and bc he may be a little dehydrated from the stress and travel.
I think it'd be great to come home at lunch, bc if he's still really young it'd be a good idea to feed him then too. I used to feed them 4x day when they first got home, then 3x a day as they became more sturdy later, and eventually (like 6-10mths) 2x day. If he does take too long to potty, you could leave a pee pad for him. Do you have a little area/x-pen for him to hang in out while you're gone? I would pen off a little area that would contain water, bed, couple toys, and a pee pad.
Just a reminder if you have a collar on him for i.d. - never leave it on when you're not with him, esp when he is a pup!
Again, congrats!