Is it odd?

Potentially, but also potentially not. Some kids are just more popular than others. My 6 year old gets invited to every party, I'm pretty sure 🤣 my older child has only ever been invited to a handful and she's now 10

I think with everything being so expensive, a lot more parents are doing the "invite up to five friends for a small party" route instead of big parties
So if he has a couple close friends and he's not getting invited to theirs it might be a little odd and might be something to check about how he behaves with his friends, but if there's no really close friends then unfortunately it's just I think what happens as the whole class parties stop being done as much

In reception my little girl went to quite a few parties. Last year she only attended 2 parties. I supposedly its down to nowadays the expense of everything. Also my little girl have made a different set of friends now in compared to reception.

Depends on what kind of parties the children are having? I know that most of the children in my son's class had a party last year and I think my son was invited to half of them. This year he's having one and we've invited all the class but next year he won't be having one, he can have a select few children over for a tea party.
It's sooo expensive, will be best part of £400 with another family meal on his actual birthday 🤦🏼‍♀️

I think we've actually all missed the point of this post....are you afraid that your child is lonely at school?
Have you asked his teacher who he plays with, who he sits with at dinner? Who did he invite to his party?