Hi ilovespinach
We had a french boy join our school last year at age 7 in a similar situation. He joined Y3 right from french kindergarten with very little reading skills or English - poor mite. Fortunately a few children in the class take a French class after school (including my DD) and so they tried to help - but 3/4 months later - it was like he'd always been there. He really did adapt very quickly.
I would suggest if you know you are going to move to start gently building up some skills to prepare for Y2 - try to look into building up phonics skills to help with reading.
BBC Alphablocks www.bbc.co.uk/cbeebies/alphablocks/
& the jolly phonics schemes jollylearning.co.uk/ are very good.
You can buy jolly phonics workbooks through amazon UK www.amazon.co.uk/Jolly-Phonics-Workbooks-Books-1-7/dp/1870946502/ref=sr_1_1?ie=UTF8&qid=1328037160&sr=8-1
and the songbook is useful too! www.amazon.co.uk/Jolly-Songs-Phonics-Laurie-Fyke/dp/1844140695/ref=sr_1_2?ie=UTF8&qid=1328037160&sr=8-2
Then - I suggest you try and read with your DS as much as possible - gradually encouraging him to start sounding out.
With maths - again simple workbooks - working on addition and subractions (start 1 - 10 and maybe go on to numbers up to 30) over the summer are a great help. We've used on-line resources like BBC Learning KS1 maths: www.bbc.co.uk/schools/teachers/keystage_1/topics/maths_ks1.shtml
also BBC KS1 Bitesize games: www.bbc.co.uk/bitesize/ks1/
You'll have to explore a bit to find things that are appropriate - but counting games (snakes and ladders) are great to just start to learn about counting on or taking away (snakes & ladders played backwards). You can also play counting games with raisins (or other small foods - grapes, sweets, etc...) - What happens if you add 1? add 2? add 3? What happens if you eat 1? eat 2? eat 3?
When you contact the school and have made them aware of the situation - I'm sure you can raise with them the question of what you can do to help smooth the transition to Y2 in the English system.
There's no getting around that next year will be a hard year for him - with a very steep learning curve - but children are amazingly resilient. I'm sure with your support and encouragement, he'll make the adjustment fine.