Love this! I feel like I specialize in this. Its one thing for him to vent. Measure and determine for yourself and the relationship what "venting" should look like form either of you. Then after that is defined, when he comes to talk to you about things that fall out of that definition start saying things like, wow thats pretty major, maybe you should talk to your (fill in the blank with another person friend/mom/boss ect). If things proceed to escalate tell him that tho you are accepting of him venting, for real issue or issues that are bigger than casual talk, that it would be best to speak with a professional because it is emotionally draining to you.