I don't think that having lots of toys necessarily equates to being a spoilt brat, I think spoilt behaviour comes more from how the child is brought up, not the number of toys they have got.
My DDs have got lots of toys, (Very generous Gparents!) but they are not spoilt. We do have very strict rules with our children. For example, if DD deliberately breaks a toy, or doesn't look after it and it gets broken, then that said toy will be thrown away and not replaced and if needed, DD will get a punishment. DD once deliberately broke a toy at the age of 2/3. It was only a freebie from a magazine, but she loved it. I made the point of throwing it in the bin in front of her. She was upset about it at the time, but she has remembered it and has never done that since. Another rule we have is that if we have something we cannot trust her with (say glitter pens) then we have put them up out of the way for a week or so and not let her play with them, until she learns how to play with them properly. As I say, she has a whole playroom stuffed with toys, but I am also quite a strict mum I'm afraid, so I won't tolerate spoilt behaviour and she knows it. She's not shy in telling her friends how to behave / what they are / are not allowed to do either!
So whilst your parents give the toys, its you, as the parent wo will decide what behaviour is acceptable or not... so don't worry about your child growing up spoilt - that will only happen if you allow it. Hope that helps to reassure.
And as for your other fear - you are your son's mum. He will never love anyone more than you. My DDs have a playroom over at my mum's house (she looks after them whilst I'm at work) and she spoils them in a way I never would, They love my parents dearly, but at the end of the day I'm still mum - and you will still be mum to your DS and no end of toys will ever change that fact. And it is for that fact alone that your DS will love you best!
HTH