Children start school the September before their 5th birthday so most children are aged 4 when they start. The best time to move is before you apply for a reception place. You need a definite address in the new area before you can apply for a school place and this may take some time. I would also start doing some research now about the demand for school places in your new area. If you are only moving a short distance away he could stay at the same nursery as this does not effect school admissions.
when is his birthday - april? if its before end of august - he will start school in september 2014 - you need to apply in jan 2014 and depending how full the schools are in your area you normally need to be living near where you want him to go school at that point - however you can move schools after they start - but if the schools are very full you may get less choice