How do shy guys show interest in a girl?

I work with this really cute guy but I'm having a hard time telling if he might be interested
We don't get to talk very often but when he does he always holds eye contact the whole time and can't keep his hands still. He always faces me directly and sometimes leans over towards me and messes with stuff on my desk. He always talks really fast too and sometimes rambles on or loses his train of thought.
But he will go up to a couple other girls and will just carry on conversations for long periods of time like its nothing.
When he talks to me he's always talking about his family or bragging a bit.
Another this is I'm the only single girl at the office everyone else is married, most with children. And I'm a bit outnumbered by dozens of single guys. A few of which have made it obvious they want to hook up with me -__-
But he's the only one i'm interested in cause he knows how to be himself and he's a genuine person.
I know he hasn't had a girlfriend for over 3 years. We text sometimes but it's always me starting the conversation. I can't tell if he's just being shy around me and making it a point to only talk to me at certain times like when I'm completely alone. Other times it seems like he avoids me. We can go weeks without talking at a time.
I was talking to him the other day and this other guy came up and started flirting with me. I tried to ignore the other guy and make him leave. But the wrong one leaves.
And I'm left with creepy dude still hitting on me.

He was talking with a close friend of his that we work with and I needed to get his paperwork so I walked up to him and his friend says, "You two act like you're in love" And neither of us said anything. I was smiling and blushing and walked off.
He's also made comments like when are you two getting married?

I don't know though its hard to tell.
How do you guys that are shy show a girl that you are into them?
How do shy guys show interest in a girl?
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