I think its pretty awful that the pre-school approached your mum to talk to them about your son tbh. I would be cross if if any of my staff did that or if the same happened to me and I would be raising it with the pre-school first thing on Monday morning.
As for the behaviour element, well it sounds to me like they need to be doing ABC observations on him, to identify any patterns, for example, is it a particular child he is pushing, is it a change in the routine that causes him to behave in such a way, is it a particular time of day etc... The pre-school should be sharing these with you on a weekly basis and gaining your input as to how he has been behaving at home or if any changes have taken place. Without the observations, it is going to be difficult for the pre-school to identify a reason for the behaviour, or a solution.
Stickers generally work wonders with young children, as well as loud praise. We often use a sand timer with one of our children who has similar difficulties. She sits with an adult for 3 minutes and at the end of it, we discuss together why she was removed from child/situation, giving her the opportunity to not only reflect on the situation, but also to develop the necessary language skills used for resolving conflict with her peers.
Social skills groups are good for children like this. All our children are taken into small groups over the course of the week and encouraged to play together at a particular thing, or to listen to each other, or to hear a social story which then leads to discussion which hopefully reaffirms the desired behaviour.
It is not always possible to have 1:1 time with children in a busy pre-school setting, and I think that if DS is rewarded with this following his undesirable behaviour then it potentially sends the message that being "naughty" results in a lovely, quiet, play time with an adult. It is possible to engage with children throughout the session and if done correctly, engaging them in an activity of interest, and talking to them about what they are doing, should hold their attention for a while, thus making incidences of bad behaviour less likely.