So he'd be going into S1 (in August) if you were to move this summer (I realise it's not likely to be as quick as that). He'd end up being in the middle of the year, rather than one of the youngest.
We have 7 years at Primary and 6 at Secondary, so he's not actually going back a year. He'd still sit his Nat 5s (equivalent to GCSEs) in S4 (equivalent to Y11).
If you wanted to consider Glasgow, Shawlands Academy is a good school and there is a variety of (nice) housing available - and much better value than the West End I might be biased
Plenty of parks, good transport links and easy access into the city centre.
It is also a School of Rugby - so the kids in the "rugby" class(es) in S1 and S2 to play extra rugby (they "sacrifice" a period of English, a period of Maths and a period of PE - but the school makes sure they don't miss our on the curriculum).
Shawlands Academy won the Scottish Power Networks Cup for the 2nd year in a row this year - for state(?) schools across the West of Scotland (and up to Stirling), combining the results of the S1 boys' team, the S2 boys' team and the U15 Girls' Team. The Final (this year against Wallace High, which is a Stirling school) is held before the Glasgow Warriors' final game of the season at Scotstoun and then they get to parade around in front of the crowd at half time.
There is a close relationship with Cartha QP Rugby Clubwhich gets a big influx of boys "new" to rugby in S1. But there are also kids who have been through the Minis section and progressed into S1. The Cartha U16s got to the Scottish Cup Final this year, playing Boroughmuir we'll gloss over the result 
Your ds would play U13 rugby this coming season, but in practice the team would consist of his S1 classmates and they would probably progress together until U16 (as you're allowed to play "up" one year group). Many of his team/class mates would technically be U12 (as in, they only turn 12 after 1 September) - this is the challenge that Scottish rugby faces with school cut-off dates that aren't (unlike England) 1 September.
If you don't want to be in either Glasgow or Edinburgh, areas with good Junior sections are Livingston and Dunfermline - but you'd need to ask on Scotsnet about the schools. Stirling also has an excellent reputation for its rugby (and Wallace High seems to be a good school. Dunblane Academy also has a good reputation).
Your ds would be very welcome at any rugby club if he is big - the S1s are usually tiny (although you really see the difference by the end of S2 as they start to mature). Good tacklers are also welcome. 
Ds was in the first year of the school of rugby at Shawlands, but he'd played since P1. He's not big though - he's a very good natural scrum half! He's just finished S6 and is off to Aberdeen (planning on joining the Uni team but also Aberdeen Grammar - and occasionally coming back down to play U18 rugby his birthday is in September, so technically he still can, although it might break the SRU Registration system ) He's also going to be looking at coaching opportunities, as for the last 2 years he has been coaching the (now) S2 team, both at school (when his timetable allows) and at the local club (U18s play on a Saturday and the rest of the Midis play on a Sunday). "His" team plays a good "fast hands", passing game - very much the SRU's "technical blue print".
I think it's this September that CBBC is going to broadcast a series about "starting secondary school" which focuses on the S1s who started at Shawlands Academy last August. They apparently focussed a lot on one boy who'd just moved up from England and was in the School of Rugby. He's a talented player. (I think there was a degree of the producers/interviewers could understand him
- one of the main rugby coaches is quite broad Glaswegian and according to ds, they had to keep re-shooting his bits
)
This has turned in to quite an epistle. PM me if you want more info.