Yes, it is against the law. However, Florida recently passed a law that keeps such persons as your son from becoming a sex offender. It is the so called Romeo-Julliette law. But, the crime the 18 year old committed is commonly called "statutory rape." I believe the statute is 800.04. Sentencing issues can be found under chapter 775.
Yes, it is illegal for anyone over 17 years of age to have sex (regardless of whether or not it is consensual) without someone younger than 16 years of age. Depending on the facts and the prior record of the offender, an experienced criminal defense attorney can sometimes negotiate with the State Attorney's Office to reduce the charge to a "lesser offense" like contributing to the delinquency of a minor or another offense that does not have such harsh penalties.
Yes, it is illegal (and irresponsible). Under Florida law a person under thge age of 16 cannot consent and any sex with a person less than 16 is a crime regardless of the circumstances. The Romeo-Julliette law simply allows the offender to avoid having to register for the rest of his/her life as a sexual offender and allows for mitigation of the sentence but does not decriminalize sex with a person less than 16 years of age.
Yes, it is illegal. Florida law says that a person less than 16 years of age CANNOT consent, period. Sexual activity with a 158 yr old by an 18 yr old would fall under Fla Stat s. 800.04(4) lewd or lascivious battery punishable by 15 yrs in state prison; 800.04(5) lewd or lascivious molestation, also a 15 yr prison sentence; 800.04(6) lewd or lascivious conduct, a 15 yr prison sentence; and could argueably fall under our state's rape statutes in chapter 794, all of which call for prison sentences and which would all call for 15 yrs or more due to the way our criminal punishment scoresheet is calculated.