I'm sorry your son is having some troubles when visiting his dad. If you feel that he isn't safe when he's with his dad (and the grandparents aren't around) - it's time for you to visit an attorney or mediator and get the visitation supervised. The safety of your son is of the most importance.

As far as your husband dating, children- especially little ones don't take the dating as seriously as we do. To them it's logical for their to be a "mommy" and "daddy" in each house, so when the girlfriend is present- this seems more natural for them.

You didn't mention what statements your child is making when he returns.  You may want to take him to a counselor (one that specializes in play therapy) to figure out what's wrong.  Your ex-spouse dating in front of the kids is not a problem (athough I  think it's ideal to wait a few months before introducing your girlfriends/boyfriends to the kids- you can't control this with him) - but you say there are other issues, so something may be happening that your child is uncomfortable with, or is unsafe.

So- outside of seeking the advice of an attorney, mediator and a play therapy counselor, here are some tips on how to talk to your son:
-Tell him you love him very much and miss him when he's gone, but it's important for him to spend time with both his mom and dad.
-Tell him that mom and dad may see other adults at time, to date, and maybe to one day marry. These relationships don't effect how much you love him- that will never change.
-If at anytime you feel unsafe- let's talk about it (Note from Shirley- if a 5 year old won't talk about it and you still have concerns, then I recommend the counselor.)

Hope that helps.

Shirley Cress Dudley, MA LPC NCC FACMPE
Executive Director of The Blended and StepFamily Resource Center
Best selling author of the book Blended Family Advice