I have an 8 year old son whom I have sole custody of. My son and myself would like to change his last name back to mine. I gave him his dads last name, we were never married. I was 19 and my sons dad was a little on the violent side, so I didn't want to fight about it so I just gave my son his last name. My son asks me all the time if he can have my last name and even signs his last name on anything he can with my last name. How hard would it be to request that his last name be changed. Because I have sole custody will it be easier? I know his dad will not agree to it. I also live in ontario

You need to call your local courthouse or consult with an attorney to get the specific laws for your area.
In Ohio, you need the consent of the biological father in order to change a child's last name. I gave my daughter MY last name when she was born NOT her bio fathers last name. When I got married I thought I could easily change it since I gave her my last name... nope... I still had to her her bio fathers permission.
So most likely, that is what you will have to do as well. And no, having sole custody doesn't make it any easier to do so.

Now you son can use any last name he wants as long as there is no intent of fraud. But it won't be changed "legally" unless you go through the proper procedures.

As your son found out, he can sign his name using your last name whever he wants as long as there is no intent to defraud. But the likelihod of getting his name legally changed without the father's conset is slim.

Once he is 18, though, he can legally change it.