T.F.
First... They are only 5 years old. When children play together it's like a group of rough rocks. Overtime they smooth each other out.
Second.... This is a part if life. It's something we all deal with from time to time. There will be instances when you feel like your child is being shafted. Those are the times you teach and help them grow from it vs feeling sorry, having the child feel like a victim or odd man out. Teach him to speak up to people when he feels like something is not right. Ex:.... Hey, that hurt my feelings.
Third.... Don't involve other parents in situations like this. Deal with your child. That makes you look not so secure and stable to parents when you address behavior of their children.
Last... You have more control if play dates are at your house. Still don't be intervening too much. The children have to learn how to work through situations on their own so they can be successful at dealing with difficult people as they get older.
They are simply learning about social interactions. Don't take it personally. I know that is hard to do but allow them to interact and develop social skills.