Causes of this behaviour? 8yo hitting, uncooperative, unruly, cheeky?

[Fleecyleesy]

Trying not to out myself but would like any suggestions as to the causes of this sort of behaviour. Just to know what help is available and from whom and whether this is something that can be helped?

8yo boy:
Listens to instructions (understands) but completely unwilling to follow them
Hits when asked to do something he doesn't want to do (eg get dressed)
Answers back to adults including teachers, not bothered about being punished
Unable to sit still, fidgets, charges around, never has "down time"
Shouts about things that really are nothing, talks all the time
Runs away when supposed to be walking with an adult or group
Is very 'grown up' despite not having older siblings (one younger) ie talks about sex, swears etc. Not mean to sibling.
Has friends, happy with them and not mean to them, although other children are often afraid of him - due to his disruptive behaviour rather than him doing anything to them
In a new place, rather than looking around and thinking, he just runs, grabs anything, climbs anything, fiddles with anything, is loud etc
Generally I'd describe him as a hurricane (but not in a nasty way)

Any suggestions? He isn't my son but I care about him and don't wish to berate him in spite of my post might sounding harsh.

Is his Dad around alot? Does his Dad spend real time with him, doing stuff?

He sounds like he needs some really good physical activity. I say that because he seems to have a lot of pent up energy that isn't being used? Outside time. Bike rides. Flying kites. That sort of thing. Can you get his Dad to invest some time in outdoor physical activities like orienteering or biking or boats or something?

Where has he learnt about sex? My dd is 9 in July. I don't think she knows anything that would be a big concern to me

To answer questions

The sex stuff is from chatter in the boys toilets at school, although he is pretty knowledgeable. (with errors, which makes me think he learnt it from other kids, possibly older ones at school, rather than him being abused)

His parents are married etc. His dad works long hours but his mum does take him to sports, park, swimming etc.