As far as I observed, the main focus of Reception in state schools is to get kids settled and socialized in a school environment. They have a lot of free play, and learning is not too structured. And, I do believe it's best at that age, as the kids in this country start schools way too young... The pace of learning picks up in Y1.
(the reason I specified state vs. private school is that boy's prep schools, for example, need to get the kids ready for exams at 6-7yo, so they can't afford to start slow. their end of year outcomes would be drastically different)
Of course, your child will not be "behind" everyone else by the end of Reception. I don't think there is a performance targets at that stage. When you get his end of year report - it will be mostly verbal description of teacher's observations. There might be words like "emerging" or "exceeding" used for some skills, but that's it.
If it's supporting his learning at home that you are after - then all kinds of activities that will help with learning phonics are helpful. I believe ORT 3-4 is about the average level at the end of Reception. In math you can help him get comfortable with numbers up to 20. Simple addition and subtractions, word problems, doubling and halving. Also - helping with practicing writing.
And finally, getting used to school is has a lot to do with learning to make and getting comfortable with new friends. Coming in mid year can be a bit daunting for a little one. Playdates can help with that.