There are more of these around than you'd think (I have one, and have found lots of kindred spirits on here) - I hope that's comforting, to start with.
Mine veers between goofy disruptiveness and a dreaminess of near-impenetrable vacancy. There are certain things he's on message for, which include maths, playing the piano, and climbing; but on the other hand he has been effectively banned from every school play since the reception nativity.
We have though seen an improvement in the last few months (he was 7 in April) and though I'm still doubting my eyes, it seems as if it might be real and lasting. The prime thing that has helped, I think, has been his amazingly supportive, patient and energetic year 2 teachers, who have been utterly consistent and positive with him, and kept a "record book" where they noted a comment on his behaviour in every lesson. He bought into this, and I think it really helped him focus moment-by-moment, and therefore learn to control his impulsivity, which has always been a problem. They also noticed where he was less able (he's somewhat on the dyspraxic side, and socially very immature) and put real effort into helping him work out how to feel integrated with the other kids, enjoy PE etc. - with the result that he's now doing a sports camp and coming back saying how he thinks hockey is great, and how his team won the swimming relay because of him - which is fantastic, all the more because it shows him being able to participate in a team of peers, which is lovely to see.
How is your son's school with him?