It's actually simpler in Scotland.
If you put him into nursery for his free 12.5 hour/week for the term following his 3rd birthday, there will be signs up in the u=nursery when the local school will be doing admissions. it's usually about January/Fenruary time. You go long with yuor birth certiciate and some sort of the residency proof, and you fill in an application form. If you want to make a lacing request, you need to apply (I think) at your "catchment school" and they will also be the ones who will give you the placing request form, which you then fill in for the school that you want.
If you are not sure which school is your catchment school (and you effectively have two: the "non-deminational tone, and the catholic one), thne ring up the one you think it is and give them your address and they will be able to tell you. Or alteernatively, contact the education department of your council.
In Scotland, the age year is from March to Februrary. There is only one entry. If your child is not 5 by the time the August term starts, who do have the choice of keeping him back until the following year. Unilkje England, he would then start P1 along with all the others (rather than skip the English "reception" - doesn't exist in Scoltand) - he would just then be amongst the oldest in his year, instead of the youngest.
If your child's birthday is between August and December, and you choose to defer, I beleive that the council has the right not to continue to pay for the 12.5 hours/week nursery time.
Whereabouts are you (roughly)?