Oh stop with the ADD/ADHD. Your child can't focus because he/she is bored! I was bored as a child because I was gifted and there's NOTHING wrong with the NATURAL human emotion of boredom. As adults I'm sure you've gotten bored before. Do you have ADHD? Certainly not! And a child doesn't necessarily know how to socially handle being bored.
Look, children don't focus on anything they deem boring or anything they don't particularly want to do. Just like adults. When the interest isn't there then of course they are bored. That doesn't mean the child has ADHD or ADD, which is becoming a blanket statement for many children who have other issues. ADHD children CAN'T focus even though they WANT to. ADHD children often get frustrated because they WANT to do something but they CAN'T focus to do it.
You're talking about a bright child who doesn't WANT to focus. A very different situation.
It may be that your child just needs some extra motivation. Or he may need a way to make learning his letters fun. I suggest looking into working with him at home and tying it into dinosaurs. There's a method where you learn to draw the letter and then the letter is transformed into a picture. I used it with my daughter when she didn't want to learn how to print. She loved drawing so we used that method and she thought she was drawing pictures when in fact she was learning her letters.
Your child may also need a motivation system. Let him know that when he can print all of his letters then he gets a reward. If he's gifted then the prize will give him motivation and I'll bet he'll learn them quickly.
Children don't see the reasons why they have to learn something and they don't always understand when we do give them reasons. So we have to give them other reasons that they understand. It's not bribing, it's just giving them a reason they can relate to. Later on they'll thank us :)
As a gifted student I often needed extra motivation because I was so bored. Teachers worked with me. I did not have ADD or ADHD, and when they finally got a gifted program at my school I flourished and no longer had issues. Good luck!