It appears that your son is suffering from viral gastroenteritis
.The vomits in head injuries are usually multiple and projectile within the first 12-24 hrs along with abnormal behaviour and convulsion
The vomits which your son was getting out was coincidental with the beginning of viral infection especially because diarrhoea
are not seen in head injury
I would suggest you to relax regarding its relation with fall.However now that your son has this gastroenteritis you could give him an antiemetic to stop his vomits and also keep him well hydrated with oral rehydration solution.