Before you do anything, find out if he is 100% single and not "seeing anyone".
Something similar happened to me, many years ago, and it turned out it was because the guy in question had a long-term girlfriend, who some of my work colleagues knew.
He used to email me from his office (we worked at the same place) all day, and was overly friendly and flirty. He would text me after work, and often spoke about "going for a drink". It was only when I mentioned it to my boss, after one too many vinos, that she said he had a long-term partner... who she was friends with.
He sounds shady. Check him out before you try to take things further.