Hes in relationship but hes flirting with me.

Hello. There is a guy... He is in long term relationship (for 13 years). We met eachother 4 months ago. From the beggining it wasnt anything serious, but for the last month or so I feel like he is getting closer to me, flirting. But hey, he is still in a relationship and I dont know how I should take him- he want just fun? No, he said no. He said there is chance for love, feelings. God, Im stuck :/  because im kinda getting feelings form him. Whats your opinion?