Assuming you may not get full cooperation from your ex on this matter, have a few talks with your son about this matter when you and he are getting along really well; don't try to do it when he's displaying this antagonistic behavior to you upon his return. Ask him what he'd like to do when he comes back home from daddy. Tell him you like him all the time, even when he comes back home and tells you he doesn't like you. Suggest to him that if he feels angry about coming home he can tell you why he feels angry. Also let him know that you'd really like to hear about what kinds of fun he had when he comes back from dad and that maybe you could play guessing games when he comes back and you'll see whether you can guess what he did with his father.
Being angry about leaving his lenient dad, and the differences in how you relate to him and set limits are certainly enough to fuel his outbursts and negativity to you when he returns from his visits. I would remember not to personalize these situational comments. Understand their origin and that he is trying to assert some authority and control in his life at this age and stage and this is one way he may feel "stronger" and more in control. Give Stephanie Marston's "The Divorced Parent" a read for some good parenting tips on this topic and others that you no doubt will face.