This patient support community is for discussions relating to step parenting, anger, behavioral issues, chores, communication, discipline, ex- spouses, family gatherings and meetings, family decisions, frustration, fun activities, grandparents and relatives, guilt, rules, stress, and time issues.
What you can control is your household and I suggest you just keep going on as normal. Your stepson will feel the stability in your household. Show him a little more love and assurance while he is acting like this. He is very young and will have limited understanding of the conflicts surrounding the adults in his life so there is not much you can actually explain to him.
I always found my children needed a little bit of time to readjust to my house after being in the other house and often their attitude was not good for the first day. Try to not be too hard on him although you cannot let obviously bad behaviour go unoticed.
Good luck I am sure he will soon realize that your house is a nicer place to be.