P1 (5 year old DS) problems

[unclehairysgirlfriend]

This is long, sorry!
My son started P1 in August. In some ways he is a typical boisterous boy - lots of running around, climbing, jumping, rough and tumble etc but everything is taken to the extreme with him - he just doesn't seem to know when to stop. Behaviour is difficult at home too - tantrums, shouting, screaming, general destructiveness - but I always hoped it would improve when he started school.
Initially he seemed to settle well but once he found his confidence his behaviour deteriorated significantly - hitting, punching, biting people in the playground. Have had several meetings with his teacher now and she says he is 'manageable' in the classroom and most of his naughty behaviour is in the playground. I asked if playground supervision was adequate and was assured it was and also asked if he could be kept in at break times and told this is not possible. Also asked if there was a behaviour specialist who could help and told that he's not bad enough for a referral but to see my GP if worried.
Started a new behaviour chart after Christmas break and first two weeks went fairly well but today he has touched another boys private parts. The little boy was obviously upset and his parents were informed. I am devastated.
We have lots of bum/willy/fart talk from him at home but I never thought it any worse than any other 5 year old boy. DD is 2.5 and recently potty trained so she's often semi naked but again nothing I thought unusual. I thought he knew not to touch anyone elses privates but obviously not! The teacher has threatened exclusion if this happens again although when I probed her on this she said I picked her up wrong and that he wouldn't just be excluded without further input.
I have explained to him tonight how serious this is and he seems to understand - it seems some of the boys have been going to the toilets together and comparing willies and as usual he has taken it too far. I should also add that this incident happened in the classroom and the boy was fully clothed (according to my son, the teacher actually didn't specify).
She asked again today if I had thought about seeing my GP but wasn't really clear on why. I have made an appointment anyway just to talk things through.
Thanks for reading, all advice appreciated!