is he cheating?

So my boyfriend of 2 1/2 years was messaging this other female he works with. I confronted him about it and asked if he was cheating one me, he told me he would never cheat on me and he was just trying to help her through some stuff because she was pregnant so i believed him and stayed with him. Two weeks later i find out he’s still messaging the same girl but i can tell that some messages have been deleted. i confronted him again and he said he kept messaging her to see if i was still going through his phone. He also asked her what she wanted for christmas and the last messages. He’s never given me any reason to believe he would be cheating on me but i just don’t know with this.