C.B.
First let me say this is not medical advice and not a substitute for seeing a doctor. That said, I'm going to comment because you've already been to the ER and have an appointment with your doctor.
In my experience, nerves can be pinched and irritated or damaged without immediate pain. The pain can take a day to three days to develop. So your son may have hurt himself much earlier, and the pain just took a while to show up. Also, all the bones may be OK in the back, yet a disk can be bulging and pressing on a nerve, which will cause it to slowly deteriorate. This will show up on an X-Ray, but most docs wouldn't be looking for a disk bulge in a young child.
If you don't get satisfying results from your doctor, visit a reputable chiropractor (there are some very good ones and some not-so-good ones, so get referrals from people you trust, or from MamaSource).
Also be certain your son is drinking enough good clean water daily. Dehydration can cause the disks to get thinner and then they can slip out of place and into nerves more easily. Notice his urine and if it's darker than a very light yellow he may be dehydrated. At 14 months he's probably still in diapers, in which case you're probably already aware of his urine color.
Another thing to check is your son's feet and ankles. He may have turned his ankle or may have a splinter in his foot. If the ankle doesn't seem tender and you see nothing in his foot, try massaging his feet and notice if there is a difference between the left and right foot. If you notice a difference, or if your son winces when you massage his feet, get a referral from your doc for a podiatrist.
I hope your son's health is good, the problem is minor, and you're all happy again very soon.