You have not broken any law by simply having your son sleep in the same bed with you as long as there is no abuse of any kind happening. You seem to be a very loving and supportive mother. You have nothing to hide from social services. You may want to try limiting the co sleeping arrangements to only the weekends and then not at all as your son is getting older.
What you can also do is sit down with the grandparents and have an open and honest discussion about how you feel about things and they need to state how they feel. Maybe there are some misunderstandings that need to be brought out in the open and cleared up. If you have a family member that can attend the conversation, then have them come as well but tell the grandparents that you will be bringing that person.
The grandparents may be upset at some other issues and taking it out on you and putting their grandson in the middle of all of this. If the grandparents do not want to talk with you, then you may want to seek professional therapy/counseling for you and your son.
If you need more advice, just ask.