Having volunteered recently in a kindergarten class for a few months, I would suggest the following:
Most classes have a workbook that pages are pulled out throughout the year for 'homework' weekly. Ask the teacher if you could take home the workbook for a weekend so that you could go through it to get a sense of what is down the road, that way you'll be ahead of the game.
The homework is usually given on Monday and is due on Friday. Realistically, it can all be done within 20 minutes sitting at the table. Break it down in two nights (or do it all at once) and get it all done Mon/Tues or Wed. evening to be able to take it back Thursday. That way there's no pressure....it's really easy.
Ask the teacher what is expected of the student by the end of the year. Here in California the kids were expected to be able to count to one hundred and identify the numbers 1 - 100, know the alphabet and each letter's sound, recognize and identify shapes - square, circle, triangle, rectangle, etc., simple adding and subtracting, identifying colors, and the biggy (drumroll please.....) TYING their shoes. It might sound like a lot (sounds like first and second grade when I went to school), but it really isn't since this is the beginning of the year. If needed, a child can catch up fairly easy at this time of year. Just work with him each day.... when you're in the car, start a counting game. On the fridge, put the shapes with colors and constantly quiz them (make it fun). Take pennies, or use your fingers and start with each hand doing simple adding, then later subtracting..... "if I have two fingers on this hand, and I have one finger on this hand, how many fingers do I have all together?".... then let them count, etc.
Working each day with one or two categories of learning will bring them up to speed and take them beyond what is expected.
Piece of cake when broken down daily....don't sweat it.... it can be a fun time for the two of you.