We had similar last year. Then it got worse. With our 'Dave' being purposefully mean and manipulative towards our DS. And stuff such as spitting in his face. I witnessed quite a few things and can only imagine what else went on when I wasn't there.
Problem was that our 'Dave' was the only boy our DS knew before starting, and DS was very keen to be accepted by Dave. Dave took advantage of this.
So we had to have quite a few conversations of the 'what can I do when Dave is mean to me?' type. Walk away. Tell a teacher. Find someone else to play with (Who?). Don't ever strike a deal with Dave because we know that Dave does not intend to honour it - only do what Dave suggests if it is something you are ok with (Dave would promise he'd buy DS something he coveted after school, if DS let him line up with his friend rather than being stuck with DS. Or would promise to buy him something if DS didn't play with DS' best friend ...).
Also we talked to the teacher and asked her to gently steer DS towards other children, which she did.
9 months on, the situation with Dave was still very fraught.
A year later though, I'm starting to think we may be over the worst. DS now has his own little group of friends and so is much less dependent on Dave, leaving Dave less scope for manipulating him. DS has also matured a lot (is now the age that Dave was (nearly) last year) and 'gets' what is happening, and has more resources for dealing with it. Last year he was totally out of his depth.
I think the key thing was building a small network of friends outside of the 'in' circle of the alpha kids at school. And that happened via play dates. Usually instigated by myself.
It was heartbreaking to watch DS being so manipulated and hurt last year. He was made to grow up before his time. I'd suggest carefully observing the situation as it develops, encouraging other friendships to reduce the dependency on 'best friend' Dave, and if necessary asking the teacher to help with this too.