I know kids, are just being kids…. But…..

There’s gentle parenting and there’s permissive parenting, that’s very much permissive if they’re not being told ‘no’ - gentle parenting still involves boundaries. If he continues to do it I’d personally step in myself and explain it’s not a kind thing to do and maybe find some ways they can coplay; if your cousin finds it an overstep then it’s on them to figure out why and that it should’ve been done an age ago x

I would have a more direct conversation and just say XYZ has been happening how do we address it?

Id be correcting his behaviour, not in a shouting telling off way. But in a no thank you, we don't do that, we're kind to people especially when they're smaller than you.
And we don't eat in the car unless it's on a long journey and then it's "clean" foods

If you really wanted to tread on eggshells say something like my little one is a bit sensitive and not able to play with your little one yet so I’m just going to keep him away until he’s old enough.

This won’t do your cousin any favours because you won’t make her aware of the problem (it won’t just be you who’s feeling this) but it’s the least confrontational way I can think of addressing it :)

My kids are 4 & 6 & their cousins are 12 & 13 & have never been so aggressive with my kids. He is literally bullying your toddler. Boundaries need to be set, that if your kids are going to spend time together, the older child needs to be more gentle with your kid or he can’t spend time with him anymore.

That's not gentle parenting. That's permissive parenting, in other words, lazy parenting. She's too lazy to parent her child. Personally, I'd be pissed. I know you don't want to ruffle any feathers, but that's exactly what you're going have to do because this kid is being a little ****head to you and your child. You need to stand up for your son and yourself.