i am single father with custody of 5 year old boy want to change his surname to

If you signed his birth certificate/registered as his (biologial) father or you are his adoptive parent or legal guardian, simply going to your local court house or registry office (depending on where you live) with the necessary documents should be all that is required. You'll have to fill out some forms, perhaps pay a processing fee and notify social security (if your child has a SSI number) and any other agencies, etc., that you have changed his name. Legally, provided you are not using a different name fraudulently or as an (illegal) alias, you can call yourself of your son whatever you wish-but for it to be "official", a legal name change is necessary.