What does medical abbreviation "hx of EtOh abuse" mean?

EtOh is shorthand for alcohol that individuals drink. The statement you posted states that there is a history of Alcohol use.
Abuse is of course a different issue which involves being involved in accidents, having blackouts, being unable to control the amount one drinks and other factors.
Should your son be not of age to be drinking this could indicate a serious problem for him.
Please talk to your doctor about your concerns.
Sincererly,
Madhup Joshi, MD
Maui, Hawaii

That's medical terminology for "history" (hx) of "alcohol" (ETOH) abuse. The doctor is saying that his/her diagnosis is that your son has a history of alcohol abuse, which he/she believes is contributing to your son's current condidition. Hope this helps.