a 18 year old boy and a 16 year old girl are sleeping together can the boy get in trouble ??

The legal age of consent to have sexual intercourse is 16 in the state of Ohio.

§ 2907.04. Unlawful sexual conduct with minor.
(A) No person who is eighteen years of age or older shall engage in sexual conduct with another, who is not the spouse of the offender, when the offender knows the other person is thirteen years of age or older but less than sixteen years of age, or the offender is reckless in that regard.
(B) Whoever violates this section is guilty of unlawful sexual conduct with a minor.

The 18 year old could not be charged with Unlawful Sexual conduct with a minor" if the partner is 16 years of age or older and the sex is consensual.

Can the 18 year old still get into trouble for other violations? Yes, that possibility exists.

If the parents of the minor do not approve of the relationship, there are other legal actions that parents can take to prevent the relationship from continuing. This is not to say they will take these actions, it's only to inform you that the possibility exisits.