Yes, it could be an attention disorder, but I wouldn't be so quick to diagnose or medicate. It could be many other things including just a neurally normal kid on the more active end of the range. School has just started and first grade can be a big change from kindergarten. It may take your son longer to adapt to the new routine. I'd take things step by step and ask a lot of questions along the way. I don't want to be too negative about meds, they help a lot of kids, but it's a big step to take.
Start with talking to the teacher about points in the day where your son can have a wiggle break. Ask her what kinds of fidgeting would be acceptable. There are sensory tools that can be brought into the classroom to help kids release energy and focus without being too much of a distraction. Can you wrap a big rubber band around his front chair legs for him to bounce his feet against (quietly, of course)? A very low profile idea is to put an inch or so long piece of medical grade tubing on the eraser end of his pencil for him to chew on. Your teacher may have good ideas from past students as well.
Is there a specific time of the day he most tends to lose focus? After lunch, during reading, etc. That can give you clues as to what's going on and how to best help him. Maybe it's the subject he's having trouble with. Or he's really smart in it and is bored with the work. Or he's sleepy after a big lunch. Or hungry because it's almost lunch.
Food: Look at his diet in relation to his behavior. Does he seem less focused after eating certain foods? The Feingold diet may be a good place to look for dietary changes. They do charge a fee but your public library probably has the book so you can determine if this is a good path for you.
Also work in good sensory foods: suck applesauce through a straw, munch pretzle rods, marshmallows, raw carrots, things that take work to eat. A google search can give you a good start. I think there's also a sensory group here so you can check with those ladies for help. Pack good sensory foods in his lunch or serve before and after school.
Activity: don't just let him run, give him what is called "heavy work." Have him help carry the grocery bags in from the car. Or push a laundry basket full of stuff around the floor. Swinging is also good. Make a kid taco with a blanket. Again, google sensory heavy work and you'll get a good list of ideas. Look into swimming lessons, karate, yoga, or tumbling for after school activities. If your son has low muscle tone, he could not have strong enough ab muscles to make it through a day.
It's funny, this is my day for sensory learning info. At the park we were talking about kids moving to be able to learn. A friend told about how her son's OT recommended practicing spelling words while stomping in a figure eight pattern. One letter per stomp. And in the movie about the girl champion speller, she jumped rope to learn spelling. Then in my hometown paper there was an article about a teacher designing stand up desks for her class. The desks are adjustable and have stools with wiggle bars so kids can choose to stand, or sit throughout the day. And now there's your question, which is causing me to write a novel. This stuff is good for everyone.
Another things that's good for everyone but most used for sensory kids is a program called the alert method or system. I think the book is called "How Does Your Engine Run?" I'll look it up and report back. Basically everyone has an internal engine. You want it to run appropriate to the situation so you're not figeting in church or dozing at soccer. The book helps you teach your son how to read his engine and choose activities that will get him running appropriate to the situation.
Good luck! Feel free to contact me offline for more details on any of this stuff. My daugher had over a year of ot therapy for mild sensory integration so I've got pages of this stuff. I'm not trying to diagnose your kid. But lots of this SI stuff works for everyone. Some people instinctively know how to adjust their internal mechanisms to make it through their day. Others need to learn the tricks.