I need advice on what to do about this guy he is soo confusing?

ok i dont want to tell al ong story so i will try to shorten this. i met this guy at work we became more than friends. he knows i have a bf who i have been with for 5 years and i was tryign to decide what to do. well he understood and he keeps changin how he acts towards me every other week. for example this week i jsut got back from vacation the whole time i wwas gone he texted me sayiong how much he missed me and couldnt wait to see my beautiful smile and comes up with all these cute nicknames for me. then when i get back he barely talks to me or even really acknowldges me he doesnt call me babe like he always does it seems like he has lost interest all of a usdden but i dont get how he could change so suddenly. he has don ethis before he was always really swee then one day he strated acting like he didnt want to be around me and i asked y he said he was having family issues and needed some time. then like 2 days later he said he wanted to hang out with me bc io always cheer him up