Can a son sue his grandmother for his father's life insurance money in maryland?

My son who turned 21 in December lost his father to lung and brain cancer in October of last year after a year long battle. His father was his main means of support even though we were no longer married and I am remarried. His father neglected to change his person on the form from his mother (my son's grandmother) to my son because it was filled out when our son was 13 and less than responsible. For the last 5 years of my ex husband's life he lived with us (myself, my husband, our son and our adopted son) in our home. We were best friends. When he was diagnosed it was myself and my husband who took him to every chemo and his surgery and almost every radiation treatment. He honestly often cried when talking about not being able to leave much more than his 20k insurance policy to his son and made me promise that he wouldn't just waste it but would use it for his future. I really think he thought he left it to him. Turns out he left it to his mom. She spent 9k on a funeral he would have hated and gave my son $700 and has since stopped speaking to him and that whole side of his family treats him strangely, because I am sure they know it is wrong to profit from this. They are using his dad's money to buy computers, pay her mortgage, buy t-shirts to sell and I believe invest in a house to rent out. My son now has a new child and school fees that this money would help with greatly. My ex left us his car (a 1995 Camry which helps but needs work we can't afford right now) and his instruments to his son who actually plays them and his mother had the nerve to call a week or two after the funeral and ask us if she could buy the car from Sam for $200 dollars and could we give her his instruments. At this point I am ready to sue her myself for his inheritance and my son said that he would do it himself if we thought it was best. The fact is we are pretty strapped right now and can't help him the way we should because of our mortgage and a new baby. It would be wonderful to not have to worry quite so much about his college tuition, his kid's future and if he could fix his car we wouldn't have to spend so much on gas etc. We live in Maryland. Is he owed a portion of his dad's estate or insurance money? Can I sue in order to maintain some relationship with his dad's family for him? They are a pretty miserable drug and alcohol addicted bunch but it would be easier for him I think. Can he sue? Any ideas or advice would be well received. Thank you