Phew! Your family lives too close for comfort. And your mum is being unfair and immature. Some mums do have favourites, usually the weaker or needier child. Or it may be the case of been there done done that!
However, for the sake of your son, you should try to improve the situation before he gets old enough to notice. Just be very rational in your approach without letting emotions cloud your judgement.
Ask your mum when she is free and invite her to join you the next time you want to take your DS to soft play, park, shopping trip etc. Ask her to come over and prepare a favourite dish etc. Try and make these occasions special so that she looks forward to them.
You can even request her to take your DS the next time she takes your nephews to the park etc.
Tell her explicitly that you want her to be more involved in your son's life and her wonderful relationship with your nephews makes you wish the same for your DS.
Give this relationship some time to develop and evolve. However if your mum continues to be thick headed, you will need to set some mental boundaries in place. Accept that she will continue to play favourites, reign in your expectations and facilitate your DS to associate with other adults - family freids, the other set of grand parents, neighbours etc.