Before reporting him to the police (?!) as other posters have suggested, the first thing you need to do is check if it's true!
I am NOT suggesting your son is deliberately and maliciously lying, but 7 year olds get confused and say all kinds of shit. At about that age, my daughter told me that on holiday her dad left her in a hotel room for hours, and him that I allowed her to cross a main road independently - I am confident neither were true. They were baaed on some kind of experience she had no doubt but in her 7 year old marvelous, creative, random imagination she had exaggerated wildly.
As separated parents, the very best thing we can give our kids is a solid, amicable, respectful co-parenting relationship. So your approach to this needs to be measured and considerate.
Approach him in the way you would like him to approach you, if DS said an equivalently worrying thing to him about his experience under your care.
You would want him to raise it, of course. But respectfully and politely. Make it clear you are making no assumptions, and just need to check what's happening as it's something DS has made a point of raising.
And if it DOES turn out he is indeed leaving DS for a half hour stretch, then yes you need to address it - but please don't report him to the police immediately. Maybe just speak honestly, say it really worries you and would he mind taking DS with him going forwards? It would make you feel greatly reassured if he would.
Assuming he is a loving dad generally, he's just shown bad judgement on this occasion but will hopefully rectify it when he sees the stress it has (understandably) caused you.
But there is every chance it isn't true, based on my experience of 7 year olds and their splendid imaginative powers.