Dealing with parents is hard for most of us and that's without dealing with the differences in "parenting styles"... I agree with perpetually sad in that you don't owe them for looking after your son. It is wonderful that they helped you and cared for him when you couldn't, but you are a responsible adult now and you need to do what is best for you kids and your family. I think its really important to establish your family and give the children the stability they need. They need consistancy and they need to know what is expected of them all the time. (My sister has the same problem with disipline and her daughter with our parents - they had to move back home for a year when they got evicted from their flat, they just moved out again and it is really unsettling for my niece)
I also think that it is important that you establish a relationship with your parents as an equal. It is really hard for many parents to do this (mine included!!!!) especially if they feel like they have been looking after you and your son (even if you were doing all the work!).
If talking to them is a trigger or upsets you, then it may be best to let hubby deal with them. Eventually you will need to talk to them - the more you can communicate the better. However, they have to understand that you don't answer to them and if you say no then its no and you don't have to explain yourself. You are in charge not them.
All the best, and let us know how it goes.