Why can't I lecture my grandson about safety without being called phobic?

I think you may have had very good intention but you got your point across the wrong way. If you would have told him that you Love him and was very disappointed by his actions as it was hazardous to him he may have understood. Though, you shouldn't beat yourself up about it because you are right he pit himself in harms way for something stupid but he is a college student and has to learn.