The truth is ALWAYS the right thing to do. It will be hard for you but it will be the best thing in the end especially for this child. Please tell this boy NOW....before it's too late. He needs to grow up secure and secrets like this ALWAYS have a way of coming out. Do you want your son to find out because a family member had a slip of the tounge? Do you want him to wake up one day and realize that everyone he has ever trusted has betrayed him by keeping the BIGGEST aspect of who he is (his biology) a "secret"?
Secret's will only harbor resentment and hurt feelings....your son may never trust you again if he finds out he has been living his whole life as a "lie". Telling him now will give you years to formulate a healthy father son relationship with him and help build trust. The longer you wait...the worse it will get. You ARE still legally his father. He has no other father (legally) besides you. You are on his birth cert...correct? If your mom legally adopted him...then she would be on his BC as his legal mother as well...how will you explain that one to him one day?
What happens when he needs that BC to get a passport or a drivers liscence or ID...or if he just finds it one day (which happens a lot to adoptee's) while riffling through some things in the house? Do you want that to be how he finds out?
There are some good books for explaining adoption to children....they can be found on amazon....search, read, and then come up with a plan. You are a legal adult now and a parent....and your actions....will have a PROFOUND life long effect on that boy for the rest of his life.....your actions will shape who is is....who he becomes.....and how he feels about himself. Everything a parent does...has an impact on a child....and if this boy grows up as a secret he will feel unloved and abandoned and trust will be zero. You have the power to give that boy security, self esteem, the ability to trust, and the ability to love....and you also have the power to take all those things away from him. Ask yourself what kind of man you want him to grow up to be.