OK first, it depends on what state he is from/in on whether the girl is underage or not....in some states the age of consent is 14.
Second, it is a fact that boys mature later than girls and some boys at 18 are not mature enough to handle the pressure of school piled on top of being away from home. Yes, even if they are staying with family.
Third, just because someone is homesick does not mean they are clinically depressed and in a fragile state. Lots of people suffer homesickness and drop out of school within the first three months...some people realize that their 'lifelong dream' of attending a certain school is not at all like the ivory tower they built in their dream.
Did you ever speak to him and ask him just exactly what the case was? I'm not sure what you needed the 'tough love' for but at 18 and an adult it would be his choice to stay or go. And it seems like you negated his feelings by making it difficult for him to go hence causing the communication breakdown leaving him no option but to arrange his own way home and then lying about it because he obviously felt he could no longer talk to you.
You must keep the lines of communication open and don't judge him for his actions but let him know you still love him and are sorry for the way things turned out and hope he finds happiness and fulfillment.
"Farts like a butterfly, stings like a bee"
REAL food rocks!!
Weight loss is only a side effect of good health!