It’s hard when a relationship comes to an end and more so when there is children involved. You are right he should have let you know about his girlfriend in advance and before allowing your son to see her without your knowing. He has his reasons for not telling you and he must make it clear from now on of exactly who your son is with or the environment he’s been put in for your own peace of mind. Make sure the issue is all about your sons well-being and safety and you put your feelings aside of his new relationship. If you are unhappy with the fathers handling of visits with your son then I suggest you seek legal advice. I wish you luck and hope you can reach an agreement in an amicable way.