I did this with a bike a couple years ago, only I was in the seller's position - if you go to the bank with him, and he pays his loan off right there (with your 5500 and his 1600), he can pay off the loan effective immediately. He can then ask for (i can't remember what it's called) "certificate of lein payoff/release" to give to you along with his copy of the title. Yes he won't have the title officially in his name yet, but the certificate "negates" that.
Then when you go to the DMV you provide both the title and the sheet, and you can get it registered and titled in your name. That's the way it worked for me and the buyer, but I'm in Maryland and I have no idea how cali works.
This is what i'm talking about
There was no wait, the bank made out the letter of release right then and there.
Honestly I would skip this bike, it's a bit of a pain in the ass. If you want to go for it, ask lots of questions at Cali DMV and HSBC