My top concern is that he is experiencing neck or back pain. This is the most common cause for random crying out and being this out of sorts. It could potentially occur from a powerful sneeze, especially neck pain. You could try to confirm if he has pain at home by a gentle exam. To test his neck, have him in a standing or sitting position. Then, gently and slowly elevate his head until his nose is pointed at the ceiling. Then lower his head until the nose is pointed at the floor. Then move his head to the left and then the right (his nose should almost touch his shoulder). If he cries out, gets grumpy or refuses then you found the painful spot. With neck pain you may also note he holds his head funny or guarded. If nothing is found then you can test his back by running a finger down his spine and pressing over each spinous process.
If you find neck or back pain, then I must recommend you have him examined tonight. This is terribly painful and can cause significant swelling which could put him at risk for losing nerve function in his legs. Therefore, starting prescription pain medication and antiinflammatories can be very useful. The sooner this is done, the better outlook for him to make a complete recovery without the need for surgery. There are no good (safe) over-the-counter pain products I can recommend. You canNOT use naproxen, ibuprofen or acetaminophen. I do not recommend aspirin because there is always a risk of stomach upset or ulceration with this and it does not provide good pain relief.
If you are unable to get him in, then encourage him to rest as best possible tonight and get him in first thing in the morning.
Aside from neck pain, abdominal pain is also possible but I could not link this to a sneeze. He would probably also have other symptoms developing such as vomiting or diarrhea. Bone pain or head pain from another source is also a possibility but I do think you would see other symptoms than the crying out in pain (which is very classic for neck or back pain).
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