What do I say?? How to show a man interest on FB messenger?

Hi. So I recently bumped into a guy I casually know. Years ago we worked for the same company, but don’t know each other well and rarely saw each other. Anyhow, when I bumped into him I was with my boyfriend (now ex-boyfriend). The guy stared at me like he REALLY wanted to speak (like stared hard enough to make me look back). Anyhow, I’m single now and would love to go out with him. The only way I knew to find him was via FB. He accepted my invite, but hasn’t said anything directly. He’s a VERY respectable guy and wouldn’t approach me if he felt I was dating someone. Also, I “think” he might be a bit shy. I’m a decent catch and guys usually just assume I’m taken. I don’t know if he’s single and don’t want to appear aggressive or desperate in my message?? What can I say??