Thanks for your query.
Your child is having symptoms of acute gastroenteritis
which can be viral or bacterial in nature.
It is not related with head bumps during skating.
Maintain his hydration.
Give oral rehydration solution,coconut water,lemon water,curd,banana,rice porridge in small amounts.
Frequent hand washing should be done.
In case symptoms persist get him evaluated by pediatrician
Wishing your child good health.