If my son is not paying his Sallie Mae student loans that I have co-signed on, is there any legal way of forcing him to meet his obligations?

You can certainly pay and then seek legal contribution from him by suing him. When you co-signed, you became legally obligated to pay in case he defaulted. At this point, you have one of two options. Either you pay and sue him. Or you don't pay and risk it going to collections and winding up on both his credit report and your credit report. You may also try to contact Sallie Mae and see if it can be refinanced in your name alone but ultimately it is his loan so Sallie Mae may not be able to do this.