Toddler seems fine after rear-end crash - what to look out for?

[BlueJayBear]

So, we had a fun father's day - a woman went into the back of us at a roundabout at about 30mph.

We're still waiting to hear if the car will be fixable (hers has been written off) as there's a suspicion that the chassis buckled in the impact.

My nearly 3-year-old was in the back, in a forward-facing seat with impact shield (don't judge, my previous car was a 3-door one and couldn't get an ERF seat). He was almost asleep when the crash happened, and we think he was sat upright (ie with head against the back of the chair) at the time.

He was obviously terrified (we all were), and complained briefly that his neck was hurting at the time, but has appeared to be entirely fine since. He is fairly articulate and wouldn't shy away from telling us if he had a headache, neckache or anything else.

He was checked over at the scene by a first responder (stayed in seat until she arrived) and she had no concerns.

So at first glance, everything's fine and we have been extremely lucky. But, as my back is getting more uncomfortable (passenger seat), I'm starting to wonder what symptoms I should be looking out for in him?

Anyone have experience with this? Obviously, at the first sign of any odd behaviour, vomiting, backache, headache, neckache etc we'd cart him off to the doctors pronto - but thus far he's been completely in character and running around like normal.