My son already get SSI but I need a copy with it on SSI on the top of the paper . They just want it to say yes he still gets it n how much. Can I just go up to the Davenport office and pick it up

You'll have to wait but if you have the proper information they will print out a letter about benefits for you. Usually it takes about an hour from the time you sign in to be called to a clerks window.

Yes, you'll likely need identifying documents, such as social security cards and/or birth certificate.