I'm a lone parent who moved from East Midlands to Scotland too. Can't help with Aberdeen as I'm still very far from there (though DD is off to uni in 12 months and may end up there so I'm taking in the advice), but I might be able to help with school differences as I found it hard to find that information.
Primary is pretty similar. It's P1 to P7 instead of R to Y6, so generally you just add a number to the English year. So P6 in Scotland is Year 5 in England. However, the dates that determine when you start are different. Generally, if you're born between September and February, you would be in the same year. If born between March and August you would start school a year later. It's also easier and more common to defer start by a year here. There's no Y6-style SATs week and instead kids just get asked to 'do a quiz' on the computer for the primary assessments (P1, P4, P7) with none of the excessive stress put on kids to do well. There's also very little homework in primary (or S1-S3). Finally, Scotland didn't have the Gove reforms so they didn't get that huge change of former ks3 content moving into ks2 around 2014. In my view a good thing, given that kids were expected to study a curriculum beyond expected child development capability.
Secondary is very different to England. A couple of years away for your DS, but I'll mention it anyway as my DD joined P4, but she was in S2 before I twigged that there is a year less secondary than in England. The September to February born children leave school a year earlier and the March to August children are the same age at leaving as they would be in England (as they went to school a year later).
Secondary years are S1 to S6, but S6 is one year, not a lower sixth and then upper sixth year or years 12 & 13 like England. In S2, kids choose their options for S3, so one year less of a broader curriculum. Some schools narrow right down and do the same number of subjects in S3 and S4, others keep things a little broader in S3 and drop another 1-2 subjects for S4.
S4 is the year kids take either National 4 (non-exam) or National 5 (exam). National 5s are GCSE equivalent, but because they are one year courses (in theory - some schools start them early) the curriculum is narrower and generally fewer subjects are done. There's only one English exam, not separate language and literature qualifications, but there are 2 maths qualifications, meant to be either Maths or Apps of Maths, but many students take both.
Kids can leave school in S4 if they are 16, but have to stay on to at least Christmas in S5 if not.
S5 is usually 5 subjects, either Highers or National 5s. Highers are sort of AS level, but as significant as A level, as kids can go straight to university after S5, but most now do S6. S6 is 3 to 5 subjects and can be National 5s, Highers or Advanced Highers or a mix of all 3. Many schools don't offer many AHs, so Highers are the key exam for most university courses. It's common for kids to get unconditional uni offers based on S5 results if they do S6.
So you'll be fine to stay for 2 years with a 9 year old, but if you stay longer, from S3 it would be harder to move back to England until the end of secondary school.