If my mother was given legal custody of my son by one state but we are now both in another state, what do I need to do from here to get custody back?

The court that signed the original orders will continue to have the case until it is transferred somewhere else.  So your starting point is with the court that entered the orders initially.  If your mom opposes you having custody, then you would need to file a motion to modify and a motion to transfer the case.  The case would have to be transferred to whereever venue/jurisdiction are currently proper (which will be where your son lives now.)  If you and your mom agree on the modification for you to have custody, then you could simply file an agreed  motion to modify and travel back to that state for a simple prove-up of the agreement.

It sounds like three states are involved at this point-- but the two main ones are where the original orders were entered and where the child lives now.  You don't have to have an attorney, but it will make the process much easier if you do have one to help with the transfer issues (which can be tedious).  When shopping for a family law attorney, find one that has already had experience with multi-state issues.  They may already have contacts in the other state to make the process move easier.