I agree.. maybe my person experience has tainted my view on this, but he's cheating. End of story. no he hasn't necessarily made out with her etc, but he's been emotionally cheating on you imo.

Cheating stems from a serious problem... it sound to me that the problem is his... that somethings not right. If you choose to ignore it, from my personal experience those things tend to get worse and worse.

I don't know how long you two have been together, or how long the ex and him had been together, but if they're working together (not sure if this is the case)... I'd definitely try to get out of the situation, even if it hurts. Ask yourself, can you really trust him? What steps would have have to take? Would he be willing to cut his ex completely out of his life for you? just some thoughts...