This is indeed a tough issue. You don't want to be overbearing with family members whom you care about, but you also don't want to compromise your own *wise and wonderful* decision to GD. Of course, in any case, your DS comes first...
I do think you DH has a good point. DS seeing your family a couple of times a week isn't going cause any huge confusion for him in the area of discipline because he will always hold you and DH as the primary caregivers, disciplinarians. Also, your DS is a smart, perceptive young man and will know the difference between you and "them."
I do think it's prefectly acceptable for you as parents, to set boundaries for family members who may be in the position of disciplining him (say, if you have them babysit DS). While I don't think it's necessary to "teach your family how to raise your child" because usually family members don't take kindly to this, it is within your rights, indeed your responsibility
to make sure that they respect how an why you discipline the way you do and carry this through when caring for your DS. If they can't accept that, then that's something they need to work out for themselves. Of course, in their house, they are will have their own set of rules (say, about what he can and can't play with) but they may not *insert any form of discipline you don't condone* him if he does something they deem "unacceptable" to them.That said, I am frustrated to no end as to why other (mostly of the older generation) family members are so unwilling to embrace new and enlightened ways of doing things? Why it so difficult for them to be open-hearted about a new approach to discipline (among other things). I am sure they did the best they could with the information they had at the time but as they wanted for us a better life than they had, don't they so wish for us to better our own children's lives? If they truly respect us and feel good about having raised us right, then shouldn't they also feel confident that we've taken the time to truly educate ourselves on the matter of raising our own kids? I suppose it's just a healthy dose of guilt and concern for perhaps not following their own instincts. This in itself, should have told them something--if it feels wrong, re-evaluate but don't hold others to your old way of doing things simply to feel you are validated in them... the emotional health of our children is at stake for goodness sake!
Sorry for the rant... this is never an easy one. My advice, while it may be wise to refrain from "instructing" do stay confident in your own convictions. If they ask, then educate. If they don't, then carry out your mothering the way you always do for all to see. I have been lucky to sway many previously suspicious family members in our direction. Or shall I say, my wonderfully, happy and secure DS has. Give it time and they may see through your DS that yes, indeed you may just have a point there!
Best of luck!