if your son is 5 in august 2019 he should start school in august 2019 very few children can read or write when they start school apart from their own name,
I was brought up near Birmingham and went to Birmingham university but now live north of glasgow, I like Glasgow and Edinburgh Glasgow is cheaper, don't know enough about Stirling to comment
Glasgow has a lot of green space for a city in fact it is known as the "dear green place" lots of culture 6 free museums and easy access to the sea and mountains and lochs
My one caveat would be it rains a lot more than England average rainfall around Loch Lomond is 2000mm per year around Birmingham it is 700mm, it is not really colder in the wetter but it is not as warm in the summer, the ayrshire coast is much drier that up towards Helensburgh
University Education is free but when I first moved to Scotland over 20 years ago Scotland was ahead of England in International tables for language and Maths unfortunately this has now reversed and the results in England are better, not many people have much good to say about the so called " curriculum for excellence"
I am wondering whether because of education we should move back to England before DD reaches secondary level, there are no SATS but teachers going on strike soon over something so in terms of education it is not all smiles but in a good school in decent area your children should be fine, just like they would be in a good school in decent area in England