Not sure if my expectations are too high here.
Ds is 3 and a half yrs old. He's always been a chilled out child, and I'm very lucky as he's very happy to listen to me or other grown ups and behaves really well. Even during the 'terrible twos' he was a breeze (not trying to be braggy but want to explain how he is). Never been a biter or one for hitting, and due to this doesn't really get how to stick up for himself.
My issue is that I struggle to find him friends of his own who aren't really really unpleasant to him. At present he goes to nursery and as he started later in the year than everyone else (and is also the youngest) we found that most mums already had a clique formed and we weren't invited to join it. The odd mums speak to me and I try to keep communication going with everyone as I want my ds to get to know the other kids too.
Anyway he has a couple of friends outside school...one of whom is awful to ds. I've tried giving her the benefit of the doubt and seeing if her behavior will change but it never does. She loves to see ds at first (talks about him all week) then about 20 mins into our play dates she starts to be mean. It will involve taking food/drinks off of him (despite him happily sharing them anyway) and if her mum intervenes she has a meltdown. The last time this happened the girls mum took my sons drink away from him to give to her dd 
and he was left drink less! I was obviously livid and asked for it back by which time she'd drunk the lot. I ended up taking him home in tears as everything he has she takes away and finishes. She pulls him off of anything he's playing with. Expects him to share all of his toys (and I encourage him to) but then won't give them back or share hers when he goes over to theirs. Unsure if this is normal behaviour for a 3 and a half yr old?
Have now started talking to another mum at school and her dd seems to have taken a liking to my ds. Seen her a couple of times and all was fine until today she decided she didn't like ds. Didn't want him to come and play near her and she refused to let him him play in certain parts of the little paved area we were talking in. Ds doesn't really understand all of it. He just stands there blankly not knowing what to do. The girl then decides to tell him she doesn't like him and he can't talk to her or look at her brother!
This seems to happen with a lot of kids he plays with. Mostly the girls. No idea why they seem lovely then out of nowhere just start being mean. All ds wants is to play with another child without it turning nasty. He's been dragged off of toys in soft play, bitten, smacked, pushed, screamed at and excluded in the most cruel ways at times. It doesn't help that ds is quite small too.
My question is whether or not this is normal for kids of this age and I've just been lucky? I would never allow my ds to hit/punch/kick/exclude or be mean and would expect him to apologise if he did.
Do they get better when they get older? Is it just a girl behaviour? All the mums say it's because "dd is a girl and this is what you get with girls" but I can't believe that all girls behave this way?
Would you intervene in any way if the mums aren't doing so? I normally try not to get involved too much unless he's being physically hit. If this happens I pull the other children away (or mine) and say "no we don't hit" quite firmly.
I really don't know what to expect with this and if it's just how kids learn to 'toughen up'. Dh doesn't obviously see most of this and just thinks I should tell ds to hit back 
whilst I don't like this idea I don't really know how else he can take care of himself when I'm not there