M.R.
D.,
You need to determine if your son has lost skills, or is developing new behaviors that are more common of a younger developmental level. If you think that he had the skill, and it is now gone, that is a much different kettle of fish than a child who has a reason to be acting out with new behavior. If your son has in any way, shape, or form lost skills, then get him evaluated by a developmental pediatrician as soon as is possible. Skill loss at any age is serious bussiness.
If you think that your son is copying behavior that you do not like, then remove him from the opportunity to copy this, and begin a consistent dicipline plan to let him know in no uncertain terms that this is not acceptable behavior for him. If he cannot respond to consistient typcial dicipline, then you need to also examine his development. Children who expereience good, typcial, consistent dicipline and cannot apply the lessons to themselves need evaluation to know what the barrier is that keeps them from learning from appropriate and consistent diciplianry techniques, something your son's cousin's parents should look into themselves.
M.