Sudden increases in aggressive behavior toward other children (such as over a few weeks) coupled with a regression in previously learned toileting skills is sometimes a sign that something anxiety arousing occurred (a trauma) to the child. This is often a stressor of some type. It does not have to be significant from an adult's standpoint; rather, it could be what most adults would view as minor. It could also be a number of other issues.
I saw many children during my tenure with a major hospital system. At three and a half years of age this type of acting out could be caused by several things.
Let's use a best practice method that offers the greatest chance for resolution. These are action steps that can be used to screen the issue.
In psychology it is never a good idea to assume that the issue is emotional. First rule of practice is medical. He should be seen by his pediatrician to rule out any issues that may be exacerbating this problem. Everything from allergies to adrenal dysfunction can cause acting out in a child of this age. Although I suspect something else, a visit to a doctor is step one.
I believe what you are seeing is what is described as a challenging child.
Challenging children require a certain approach. They tend not to respond to the typical redirections and punishments that quickly assist other children. About 3% of children begin to develop challenging behaviors and limit testing between 3-5 years of age. They are usually boys and are above average in intelligence. As adults they are dynamic and creative, often leaders. As children, they are in a word, difficult. Their brains at the ages of 3-7 lack the maturity to control their impulsive behavior and they often get into difficult social situations, particularly in school. With time this matures out (around age 8-11) but in the mean time this can be a real parenting challenge.
One of the best approaches for a challenging child is the use of strong and consistent behavioral modification. Behavior modification techniques work, are well researched and are safe. The best way to learn these skills is through a resource such as:
Connected Parenting: Transform Your Challenging Child and Build Loving Bonds for Life by Jennifer Kolari (Hardcover - May 14, 2009) Very inexpensive on amazon.com
Also: The Difficult Child: Expanded and Revised Edition by Stanley Turecki and Leslie Tonner (Paperback - Mar 14, 2000)
And: From Chaos to Calm: Effective Parenting for Challenging Children with ADHD and other Behavior Problems by Janet E. Heininger and Sharon K. Weiss (Paperback - May 8, 2001)
I would start with the third book and then book one then two. The methods used by the books should complement this reading pattern. (these are not difficult books to read or to enact the behavioral guidelines.)
If these resources fail to help (I think they will help tremendously) an evaluation by a child behavioral specialist is in order. The pediatrician will be able to make this referral and the child specialist will work with you and the child to develop a customized behavioral plan. A rule out of any other contributing psychological factor will also be made.
Take heart. this is most likely a sign of a dynamic and intelligent child who is struggling with impulsivity. The behavioral resources I recommended will help and he should be seen by a pediatrician just to be sure nothing else is affecting him. Steven