DGS1 is only 2, so I shouldn't be worrying about this, but ..... His local school has an Excellent ofsted rating, and is very popular. Does this make a difference when it comes to applying? My son and his family are in England, I'm in Scotland so I'm not sure if things are different there.
If your DGS is in England, he can start in reception the September after he turns 4. (Although his parents can choose to delay entry if they wish until the term after he turns 5).
His parents will need to apply for his place in by the January before this. They apply through the local authority, and have to put a number of choices down. They should visit a few local schools, not just the one they hope to get. You can't apply early (assuming you are talking about state schools).
I forgot to say.... The fact that the school is popular doesn't really affect the application process. Places are allocated according to a strict set of rules published in advance. The parents can start looking into these to see how likely they are to get a place.
They usually decide in order of catchment and then siblings. If you live in the catchment area you should get in, unless its massively oversubscribed by locals obviously but you can only apply and see how you get on.
I did mine online. I think the website opened in November or December, and you have to have applied by the cut off date of mid January sometime, they will let you know when this is. My children went to the preschool attached to the primary school and in the year before they start, I had a letter from the school, as it was our catchment, inviting me to apply for a place. You don't find out till mid April if you got in or not. You will probably find all this out in a bit more depth nearer the time. I think your local authority will notify you wherever you go to preschool, as long as your child is registered locally obviously.