If I’ve had full legal custody of a 6 year old for almost 6 years but his birth mother has informed me that she is now seeking to get custody back, what steps should I take to oppose this?

If you have had full legal custody per a court order-- all that you need to do at this point is just keep being a good parent. Considering that she has abandoned the child for six years, she's going to have a hard time explaining why custody should be removed from you.  Stay focused on the things that make the child healthy and happy.  If she does decide to file a custody suit, you'll already have several examples of why you should retain custody.  You can't stop her from filing a motion to modify custody, but you can gather and develop evidence that will help your position.

If you have custody, but you don't have custody per a court order-- you really need to get one so that she cannot come and take possession of her child. 

If she does file suit and she has not provided any support for the child and has nominal access to the child, you may also want to consider filing a counter-motion to limit or terminate her parental rights.  Sometimes the counter-motion will be enough to discharge sudden interest by absent parents.

You are not required to have an attorney to help you through the process of defending a custody suit or filing a motion to terminate... but it helps.  Until she does decide to do something, you may want to use the time to shop for a family law attorney and get an idea of which attorneys accept payment plans and the costs for representation.