How can i protect my son?

I dont think your grandparents have right to apply for custody of any sort- they would have to prove you are an unfit mother before they got any custody rights, which from the sounds of it will backfire on them anyhow so I dont think they would bother. You also have the step mother on your side and as a witnesss if it does happen so I cant see it going any other way than you having full custody.