Yes, it has 1x PCIe 16x slot so you can put that graphics card in.
Also, if you are using the 550 Ti for everyday use and very basic gaming, it's fine. But if you/your son is looking into gaming at a decent level or working with graphically intense programs, I would recommend a HD 6870 as it will provide better all round performance. (http://www.newegg.com/Product/Product.aspx?Item=N82E16814150561)
Also, I would say it is a good upgrade for only an extra $45 or so and would save you the extra money buying a whole new graphics card in the future. But don't get me wrong, the 550Ti will handle most things you throw at it but the 6870 is a good choice for the money. Hope I havn't repeated myself alot, haha.