In general I would say it's normal - my daughter is like that as well. She simply is a dreamy person, that's just her personality. Makes her creative as well. Since she's never disturbed anyone (like your son, she looked out the window or things like that) and did well at school, I wasn't concerned.
However, since your son is behind, that's a different issue. SO I would certainly try to increase his ability to concentrate - actually the summer holidays are a great time for that, because you can implement some things at home during his time off.
I'd start with thinking about his usual day, or his life in general. Is he being entertained much? By you or anyone else, by TVs, computers, etc? If children are being entertained a lot, they don't learn or unlearn how to entertain themselves and get bored easily. This might be a cause for you son quickly losing focus as soon as he doesn't find something very interesting (= isn't being entertained by his teacher).
Another thing is distractions: In order to learn to concentrate on something and really focus, a child has to be able to do that without being distracted. Distraction can come in many forms - background noise from a TV, radio, etc. Siblings or parents who are disturbing the child while being concentrated. Too many toys, a too crowded play area. Just some examples.
Now ideally these things should be avoided from early on, basically from when they are a baby, but of courses changes can be made at any time.
What you can also do - additionally to the changes above, or if the above mentioned things don't apply to your son - is get him to do activities that require attention and concentration, so that he can slowly increase his ability to concntrate.
That could be board games which require a certain amount of concentration (e.g. memory), drawing (free drawing or also coloring books - there are coloring books for older children and even grown ups with a lot of small spaces), balancing, climbing, chopping vegetables, ....
Did he recently start with the violin? An instrument is really great for concentration! Make sure he practises daily and slowly increase the time he has to practise at once.