It really does depend on what 'a bit behind means'.
DS1 was very very behind on his reading and phonics, however he did have a speech disorder and has since been diagnosed with dyslexia. He still finds reading and phonics exceptionally difficult but has 4 A' levels grade As and A*s and is studying for a maths degree.
DS2 was also 'behind' throughout reception and year 1. At some point in year 2 it just clicked. He went from being in the 'cause for concern' group to reading well above average in a couple of months. He was assessed at secondary school as having a reading age 2 years ahead of his chronological age.
Lots of work on discriminating sounds, hearing initial sounds in words, listening to and identifying rhyming words, working out words when you phonetically break them down for them, listening to you read to them, listening to auditory books, plus activities the school send home will help.
Blending sounds is a really difficult skills lots of exposure to it will help but make sure you are using the pure phonics sound (so 'b' not 'buh', 'mmm' not 'muh')
Some schools subscribe to phonics schemes which have specific resources for home. It might be worth asking.