I think Cailyn's idea is really good, to do the housecheck. I think maybe the fact you are being a lovely mum and taking his fears seriously makes it seem that his worries must be more 'real', iyswim? Possibly?
I remember one mum on here, having trouble with a really tearful, clingy dd at pre-school, had been trying all sorts of things to make her dd able to be left a bit better, gave her incentives and praise etc...had NO effect whatsoever; then one day the mum simply stopped discussing it, stopped giving it any importance at all, and the dd was kind of taken aback, but her ability to be left improved no end with that approach I believe.
I don't mean ignore your son's worries, maybe just try a slightly more robust approach, just tell him "oh well, we've checked the house, all done!" end of story, even if he tries to go on about it.
This might sound an odd approach but it works with me too when I am getting anxious - I always get paranoid about my health when I'm really stressed, and go on and on to dh about "maybe I've got cancer/TB/the plague"; if he says, "well, go to the doctor and get it checked" my response is even more anxiety, feeling more scared and like I'm right to worry if HE thinks I ought to go to the dr.....if he just doesn't engage with me and says "oh fgs woman you are FINE" then I might witter on for a bit but it does put my worries where they belong iyswim.
Sorry to blather on but I think it's a difference in approach that could work for you with ds.