Hi goatshavestrangeeye:
First off - here www.satsguide.co.uk/sats_resources_area/sats_levels_results_and_timetable.htm is a very brief guide to the SATS levels at KS1.
Basically 2b or above at the end of Y2 (around May/ June time) is national expected level.
So that being the case your DS is doing exceedingly well in reading and writing, but may be grasping maths a bit more slowly, but is more or less on track. In general children progress 2-3 sub-levels per year - so working at 1a now puts him more or less on track for 2c/ 2b in May.
But yes, it sounds like you might want to start to notice how your son is doing with his basic number facts to 20. It may be that he's struggling with addition and subtraction once he's working with numbers over 10. My DDs both struggled at this stage.
Try something very simple. Try rolling a single die and subtracting those numbers from 10. If that's going well, try a few more two-digit numbers (maybe 11, 12, 13 & 14. Then keep working up to 20. It's sometimes helpful if you have checkers or legos to set out the number your subtracting from (so for example if you are taking 5 from 20 - set out 20 legos and then have your DS remove 5). Sometimes the link of visually seeing what is happening and writing it really helps.
Y2 is also when the first times tables are introduced - usually 1, 2, 5 and 10 are introduced first. Again, some children (and definitely mine) have initial difficulty linking multiplication with multiple addition of the same number - so 4x5 = 5 + 5 + 5 + 5. Using legos or checkers does help with this visualisation early on.
If you feel the issue is speed and what he requires is more practice - I heartily recommend Tux of Math Command. You can set it for ability level and just work through addition, subtraction, multiplication, division (inverse of multiplication) slowly and step by step. You can find info on Tux of Math Command and how to download here: tux4kids.alioth.debian.org/tuxmath/
Finally, BBC KS1 Bitesize www.bbc.co.uk/bitesize/ks1/ - has a number of videogames to help review many points of the national curriculum in advance of SATs.