Welcome to the board! A turtle is a wonderful pet to enjoy, though there are some considerations you should make.
First, RES are not native to Maryland, so he is likely an escapee. Being so small, he is also likely very young. If there are no known turtle water areas nearby, and he's friendly towards you, he's probably captive-bred. If you would like him to stay, you may need to build walls around the pond to keep him from wandering off. If you are in a suburban area, he can easily be hit by a car or taken by a predator without protection outdoors. You will also want to provide something for him to bask on (such as a rock) and something to hide in if a predator shows up (like a large hollow tube or pot underwater). Predators can include local dogs, birds of prey, neighborhood children, wild animals.
If your pond is chlorinated, you may want to consider dechlorinating it. The chlorine can irritate their eyes and he may not want to stay around because of that. Hibernation is not suggested, though if you can get an indoor setup inside, you can move him in there when it gets cold out. Esp. since your pond only half-freezes. Turtles' metabolism is based on their temperature, and if they are cold enough, they will stop eating, but if they aren't cold enough to hibernate, they'll stay awake, basically starving to death. It sounds like your pond isn't ideal for hibernating or remaining in, so he'll need an indoor place eventually.
If this doesn't sound great to you (and don't feel bad, we get a lot of people whose lifestyle this doesn't fit into), it would be best for him if he was caught and given to someone. I would suggest first asking on the board here if anyone local wants him. RES are overpopulated and hard to place. However, you could see if the National Zoo would take him, esp. since he seems to be wandering around homeless. I saw they had a nice pond that was full of turtles there, so they might be willing to take him or find somewhere for him.
If you do want to keep him, please ask as many questions as you need! The main site here,
www.redearslider.com, has a ton of information on it.