Craicon That's sad for you. I am in same position, even with the foreign inlaws, but my DH's late wife died 6 years before I met him, and now I have been with him longer than his late wife. 25 years! The boys have known me since ages 18 and 21. Of course, they call me by my first name.
The older one has our DGD now and he and her mum asked me which version of grandmother I wished to be called!
We are very involved in their lives and being called grandma (or gramma, grammie), even in third person by them just melts my heart.
Has your husband told his son that it's also about respect, not just an endearment? If you have a good relationship with DSS, he might be urged be more considerate of your feelings. 