OP, I get why you are upset about this. I am a widow too and it's difficult sometimes to keep an ongoing relationship with the other side of the family when that main connection is not there. It sounds like you did your best to do that, and that warmth and love flowed from that side, but it also sounds like they perhaps didn't think through the will and how that would play out, or they did and didn't take action to include your son back in.
I agree with everyone you aren't owed anything, and that you should get a copy of the will for your own satisfaction and for legal clarity.
It is hard though, you are essentially imagining what might have been, had your husband been alive. It can be hard, then to accept things on behalf of your son who you feel very protective over.
You also, as my children's grandparents did, know that if a child is bereaved like this, often the family do rally around or make monetary provision if they have a lot of money because that's something that they can do in the face of losing their own son- invest it in the next generation. Sounds like that's what your FIL was thinking at the time, and if they have trusts they must have been reasonably wealthy and so that is not an odd thing to think.
One of my relatives made verbal promises that didn't materialise in their will. I don't even think it was deliberate, you are right, acceptance is the way forward.
Don't let anyone on here make you feel bad, you sound like an amazing mum who kept going when you were bereaved at such a young age, and you are a protective mother of your son. You are more bewildered than trying to do anything concrete here, just making sense of it.