Hmmm, that's an interesting one. Sounds like you've done all of the things I would do, too. Have you asked him if he stepped on anything that hurt or asked him questions to try to draw out anything unusual over the last week or so? Did any routines change recently where he may just be more tired or subdued than usual?
It may be that he hurt his foot a little and is now experimenting with what it feels like to walk on a different part of it. Kids can be the great experimenters sometimes. But keep watch and trust your gut.
If you think something is persisting, then I would get it checked out in a day or two. But give it time. If it were something dire, you would hear more complaints from your son by now. However, if it recurs periodically or he complains of it more, I would definitely check it out.
One time our 12-year-old complained about his knee intermittently during soccer season (several years ago). We didn't think anything of it at first and all the things we checked didn't seem to hurt. But he complained intermittently over several weeks. My gut said it was worth checking further. We ended up having an orthopaedist check it out and our son had some issues on his growth plate of his knee. Nothing serious but something recognizable by a doctor.
Trust your gut. If it persists, then it doesn't hurt to have it checked out. But also trust your gut if you think it is something small that will pass.