Just settle in and try and engage him, offer some treats, scoop him up, skritch him, follow him around a bit or even play a little chase or toilet paper tug of war... it really depends on the rat... Don't be overwhelming or overbearing but it's OK to get his attention so he understands you aren't furniture...
Basically it's engage him, then see how he responds and then try to reply to him... try to get a dialogue started.
I know it's hard because your rat is small and you are large, this would be easier with a puppy you can chase around the yard with, but it's the same general principle. Another analogy is working with a kid from a foreign country that doesn't speak your language(s). You use body movement, vocal tones, treats hand signals and whatever it takes to get your rat to understand that you are very much like him... He has to understand that you are a sentient being too then he will begin to try and understand your meanings and be more interested in interacting with you.