Children this age, and older, are very fickle. From moment to moment, day to day, they will be friends with someone, then the next time not. Or they will explore playing with others. And sure, temporary likes and dislikes will occur.
This is how they learn about socialization and friends, and what a "friend" is. It is an ONGOING experience for kids. Even for teens.
My daughter, at that age, had friends or kids she liked and played with. Then the next time, maybe for some reason, she decided another kid was her friend. Its all about them 'learning' about people and what they find nice/or not, friends or not.
Its not always consistent. Their feelings/emotions at this age, is also STILL developing. So, its bound to happen that some kids may from day to day, will not want to play with another. My daughter had that happen too. But, she shrugged it off... and just went to play with someone else. She didn't take is personally.
If your child is not bothered by it, or not saddened, then that's fine. Or talk with him about it, about how all people/kids are different... and not everyone will like everyone, but the main thing is to help him cope with it. Or if someone does not want to play with him, then talk about it....and teach him that he can play with someone else too, or HOW to make friends with others.
If he is fighting with a child... then you should tell the Teacher. MANY preschool Teacher's, will incorporate into their lessons, about HOW to treat others.... so speak to the Teacher about it. They are there to guide the children... not just let fights occur. At this age, and other ages, this sort of thing will happen. And, the Teacher should teach the kids about manners etc.
And yes, even at this young age, kids can be "cruel" or mean. So, just teach your son about "character" and how to discern... so that he can (later as he gets older), be able to choose wisely and navigate himself... or find other friends without fighting. That is what we do with our kids.
But if there is trouble or fighting, or someone "bullying" them, then teaching them to seek out help...ie: telling the Teacher. That way they will learn HOW TO handle things in the future. It takes time. But it will create a confidence in them. My daughter as a result, now at her age, can really pick friends wisely... and she really is a good judge of character. I can actually TRUST the friends she hangs out with and her choices and judgement.
Boys ALSO need to learn how to communicate their feelings... and that it is okay. Teach him how to express himself and what he is feeling... then that way, in the future, he will feel comfortable in telling you things/what is bothering him... and then how to say what is bothering him to others as well... instead of fighting, or seeking out help for it.
In each grade level they become, they will attain different friends. It can be transient, or not... but main thing being, they can flex and flow with it... and express their feelings about it.
And, if he does have a consistent friend, then you can foster that by having play-dates with the child/Mom. That way he will learn about what friends are, and how to socialize.
Juts some ideas and what we do with our kids.
All the best,
Susan