Thank you for the additional information.
It sounds like your partner may have a jealousy issue. From your description, you have done all the right things. When in a relationship, it is ok to have friends of the opposite sex as long as you include your current partner in the relationships. That means introductions, invitations to be a part of get togethers and keeping your partner fully informed of any contact. If you have done all of those things, then it is your partner with the issue.
Many people get into relationships thinking that their partner will love them unconditionally and that any problems they had before will improve or even disappear. When they find out that the person they are with as their own needs or wants, their issues come back, sometimes even worse. So if your partner was insecure before, it will be worse now especially because he now has a "reason" to feel the way he does.
It may help your partner for you both to sit down and talk this out. Let him know that you understand his problem and you want to help. But he needs to see that he is part of the problem and it is not all about you and your choices. It would help you to talk with him if you know more about insecurity and how it affects relationships. Here are some resources to help you:
Once you know what your partner's issues might be, it will be easier to talk with him about working on his problem so he can feel better about himself.
You may also want to try making some special time for just the two of you. Focus on each other over dinners, special trips, and other activities. Surprise him with a homemade candlelit dinner or take him to a place he loves. It will help him feel more sure that you love him.
If your partner feels he cannot work through his jealousy, he may need to see a counselor. He can find one through his doctor or search on line at http://therapists.psychologytoday.com/rms/. His issues may be deeper than he can handle on his own and a therapist can help him learn why he feels this way and what he can do to help himself.
I hope this has helped you,