I'm a teacher and I agree it's not ideal to swap at the end of Reception year but some schools choose do it once they've got a feel for personalities and others will do it on a needs must basis if there are specific issues within a class/year group.
It also helped that my son was dominant and his friend more submissive so happy to be in my sons shadow so to speak. Not sure that that dynamic was a healthy one but not sure my son is finding it easy to relate to the other children where the dynamic is different.
This jumped out at me though.
When classes are mixed, a lot of consideration is given to friendships, the new mix, personalities, needs and making sure that children have at least one friend etc. It's not done randomly.
It sounds to me as though they deliberately split your son up from his friend (rather than it being an accidental oversight) for both the benefit of both boys.
You say yourself it probably wasn't a healthy dynamic. It's not appropriate for the other child to live in your son's shadow and your son needs to learn to develop friendships with children on a more equal footing.
The other child's parent may well have raised concerns with the school about the friendship and the school may have shared those concerns.
If you suspect ADHD, then make an appointment with the SENDCo to have a conversation and find put exactly what behaviours they are observing at school.
Make an appointment with your GP and ask to be referred for an assessment. The school will be contacted about this and support the assessment with their own observations.