TBH if my ds got caught talking to another human being like that he'd get more than a smack (and I wouldn't feel bad)!! He'd get the smack and a punishment and made to apologise to the person the next time they saw them. He'd then get spoken to and made to realise that if he ever did it again, next time's punishment would be ten times worse.
In fact, I've had a similar situation with DS listening to bullies and being made the scape-goat. I've told him that if he EVER behaves in that way again, he will to face my wrath, quite frankly (I put it in terms he will understand of course!)
From that day (of the discussion - in January) to this, he has not once stepped out of line on that issue and has started to stand up to the bullies.
Have you tried the "ton of hot bricks" approach and made a huge impact on him? I find that every once in a while issues arise that need a short sharp shock punishment/reaction and then the message gets through.
Also, if you say he'll face his dad, then make him face his dad - I remember being told that my dad was going to get a report on how I'd behaved each day at about your DS's age; it didn't half make a difference!!
When you say Friday afternoons are "nearly" trips to the park and snuggles etc does that mean they don't happen? I think that if you say something will happen - even if it's just a DVD or something, then it should happen to stop your DS learning that threats and treats are just empty words(?)
Could you do a sticker chart that every day he comes out of school with a good report, he gets a sticker - he needs 3 out of 5 for the first week to get a treat (sweets/magazine - not something huge) then by the end of the month, he needs 5 out of 5 and will get a big treat (cinema/bowling etc)?
I put DS to bed at 6 30 most nights and find that any later than 7pm is just not going to work - he usually goes up at 6 30 and is actually asleep by 6 45, so perhaps you need to bring bedtime forward some more? I don't know - only you know how much sleep your DS needs.