I don't think you're going to like my answer. LOL.
Toy should never be hung from a child seat handle when the vehicle is in motion, and while the use of a mirror may not be the worst thing in the world, unless the mirror is super-lightweight and flexible (basically a cloth pad around a reflective sticker), it's way too heavy and dangerous to use in the car. The strap on the head rest cannot and will not hold it there in an accident. A mirror that lights up is probably from the 'heavy plastic and therefore dangerous' category, although I concede there is a product out there I'm not familiar with.
Anything that you put in the back seat with your infant should be something that you'd be comfortable hurtling toward his head at 30 mph, because that's what would happen in a severe accident. Small, soft, plush toys are probably fine. At three months old, his fine motor skills are not developed enough to really interact with things beyond generally feeling them anyway. Some kids do well with some sort of pacifier in the car, even if they don't use them elsewhere.
Is the seat comfortable for him and is reclined to the correct angle? What seat is he riding in? He's not wearing a snowsuit or thick bunting, and there's no "Bundle Me" or similar product in the seat, right?
Three months is a tough age. My kids were miserable in the car then, and I avoided car trips as much as possible. Then they were good for a few months, and then disaster struck around 13 months again...lasted until around 20 months, and we've been good since (they are 7, 5 and 4 now, and no permanent psychological damage ;-) )
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