Tell the children that you ex still them and it wasn't their faults. Call your ex to come over to sit up with you and your child and talk about the child being naughty.
Could be he is now jealous , if you are the sole parent now most of the time he gets less attention, so make sure he has one to one time with you,its very hard on children when one parent suddenly isn't around all the time , make sure his Dad is part of his life if its possible, children with two parents in their lives do better .good luck
Have you tried talking with him not really you talking but more listening to him make him feel comfortable enough to talk with you just you and him without any other distractions and when hes in a yulnerable state i knwo that my son who is seven as well my daughter will open up to me when they are sad or upset but i make sure that they know my attention is solely theirs in their own rooms cause tht is their space so they should feel more comfortable or even when you are tucking him in.. . Also talk with his father if he is a part of his life and ask him what his behaviour is for him All I know is it has helped at timesto allow them that time to talk about whatever it is that they want to talk about it my children are from perfect but for that bit of time following that talk time they seem to actually appreciate me... just a suggestion...