My ds is identical to this except he’s not especially bright - or not in the way school measures “bright” - and I don't think it’s adhd… it’s being 5 years old that’s the issue.
The fact you say he can concentrate when he’s interested tells you everything (my ds will play Legos for hours, for example).
Year 1 is a huge step up from Reception - all of a sudden it isn’t free play any more. Suddenly, they have to sit and listen and do certain tasks when they are told to.
And classrooms are noisy, interesting places with 30 fidgeting bottoms and large windows looking onto the outside world.
I agree with pp that ds needs to practise focusing. I’ve noticed since I started doing the (mountains of) homework set by my ds school, how this has improved. In Reception he really struggled to sit down and do anything for more than a few minutes. Now he is in a routine, all I have to do is I gently remind him to sit on his bottom, put the pencil down while we are talking, etc so he remembers how to sit quiet and still.
My ds can now work for 40 mins at home without tiring, which is ample. But that’s only at home where it’s quiet and there are not a million distractions!
We talk to dc about good focus which means having your eyes on the teacher when teacher is talking to the class, and ignoring noises or movements around you when teacher is talking. It is worth explaining to ds that this is the expectation. And then practice at home with some quiet supervised work?