Speaking from personal experience .As I was growing up I had several coordination problems , which all stemmed from my body growing at deferent rates , my arms suddenly longer than my brain recognised so I was forever knocking things over as I reached out .Falling down stairs as my legs grew .Reaction times being affected as a result . Not saying this is your grandsons problem but it could be a factor .
Could also be just one of those days when as an adult you know after the first couple of shots you know its just not going to be your day and all you do is get more frustrated as you keep missing .With my own kids I do know that if they felt they were failing or not doing well they would want to " chuck it in " .Give him time and hopefully he will come back to it .