If my boyfriend wants our baby to have his last name, what problems will we face if I’m 15 and he signs the acknowledgment of paternity showing him as 21?

You may run into some legal problems with your boyfriend being twenty years of age.  Statutory rape laws vary from state to state, and the age of consent is different depending on the state.  In Texas, a person is considered able to consent by the age of seventeen.  Some states have a close-in-age exception which means for example, if the minimum age of consent is fourteen and there is a three year differential, a fourteen year old could possibly consent to sexual relations as long as the other individual was not more than three years old. 

Additionally, statutory rape in Texas is considered a second-degree felony and an individual who is found guilty of this crime can face up to several years in prison.  Since Texas categorizes this crime as a second-degree felony, and individual convicted of this crime could possibly face up to twenty years in prison.

However, under Texas Penal Code, Section 22.011(e), there can be an affirmative defense to this charge if all of the following apply:  (1) the person accused was not more than three years older than the alleged victim, (2) at the time of the so-called “offense”, the alleged victim was older than fourteen years of age, (3) the accused person was not registered as a sex offender nor required to be registered as a sex offender at the time of the alleged offense, and (4) the conduct at hand would not constitute incest, and lastly (5) neither person was married to someone else at the time of the act, so that neither could be accused of bigamy.

Since this crime bears a potentially serious prison sentence, it would be helpful to contact a criminal defense attorney in your area.

You are very wise to have some forethought on this at this point in time.  He is an adult and you are minor and regardless of what the two of you say, he could be in a whole lot of trouble.  Sex with a minor is statutory rape.  Plain and simple and no two ways about it.  Even if your parents do not file a complaint - which as you guardians they are entitled to do - the state has a right to prosecute,  Consent would be his defense but it is unclear that it would matter.  It is admirable that he is doing the right thing here.  But you should speak with an attorney in your area familiar with the rape lawsin your state.  Good luck to you all.