You’ll apply between October 2026-January 2027, ready to start September 2027.
He could start school nursery if there is one, which would be sept 26. I know some schools do a January intake though so that could be Jan 26. You apply directly to the school for nursery. You’d apply for reception place towards end of 26 (deadline Jan 27) to start in September 27
Are you in the UK? It’s September start. Many years ago, they did half year starts (I remember I started in September but the younger half of my class started in January) but that stopped a long time ago
Im sure they start the September after their 4th birthday. So he will start September 2027 and I think you can apply from the September before x