My son is now 2 and his dad has seen him maybe 2 times since he was 4 months old. When I gave birth I thought that we were going to be together but things changed. I gave my son both of out last names, it is hyphonated. He has my last name first then his fathers. I am not sure which one is his legal last name if he can just go by one of them, or does he have to go by both. An I was wondering how and if I can change his last name or do I have to have the fathers consent?

Where are you? (what state/county)
I know the laws regarding this are different, depending on where you are.

However, I'll tell you what it is in California, as a reference.

If your last name is hyphenated, then, for all legal papers (birth-certificate, social security, etc.) Then you MUST use his full last name.
For things like school, depending on the requirements of the school district, you may have to register him by the full last name, however you can talk to the principal or someone and have just your name on the roll sheets/etc.

To get his name changed, if you have joint custody (even if he is only entitled to one weekend a year) you must have his consent to change it if he is under eighteen years of age.

The hyphenated name is his legal name. If it's Smith-Jones it would be the same as if it were Smithjones. You can't just lop off part of it.

You would need his consent to change it.

What ever his name is as listed on the birth certificate is their legal name.

You can call him anything you want, but what is on his birth certificate is what they have to use to enroll in school and drivers license and social security card