Your son needs to develop a code of ethics which uist come from the parents as to what is right and wrong. Chidren under the age of 4 and 5 do not have complete concepts of ownership. They really do not understand that it is wrong to take things from others.
Your son may need to develop more self control. Allow your son to get what he needs in a honest way. Make him put objects back and ask for them if he wants to play with them. You must be consitent with this so that the code of ethics can be developed.
Maybe your son's basic needs are not being met as you might think they are. A child who feels like his needs are not being met will eventually take the matter into his own hands and take whatever he wants or needs. Are you more attentive to you daughter. Your son may be feeling an emotional lack in his life and feel empty inside and may take things to fill the void. He lacks the tools to express his felings.
When your child steals:
Stay calm. Do not over react.
Do not take it personal. He is stealing for attention.
Make sure your child know what he did wring,
Be a role model for your child.
Give him more responsibiities at home.
I hope that this has helped. Please ACCEPT so that I may be compensated for my efforts. Thank you and have a great day.