There is a stage before blending which they need to grasp before blending will make sense to them - it's called phonological awareness and is partly the idea that letters or groups of letters can represent sounds, and partly the idea that words can be broken down into separate parts. My DS nearly 7 (with SEN) is only just grasping this, and can't blend at all yet. There is basically no point keeping on about blending if they haven't got there with phonological awareness yet.
Some useful games to practice phonological awareness are anything with rhyming - if he can't identify rhymes himself yet, then lots of exposure - songs, nursery rhymes, funny rhyming books (especially books which use made up words, like "Octopus Socktopus").
Also beginning sounds. So things like I Spy (using phonics sounds rather than letter names) if this works, or if that's not working yet, you can work it into your everyday play together picking up objects and asking, or when he wants you to spell something for him, ask him "Does 'go' start with the sound 'gg' or 'lll'?" And once he's consistently picking the right option out of two, you can start to get him to identify the starting sound without offering multiple choice.
If even multiple choice is baffling still, you could work on specific sounds one at a time (maybe go through the sounds in the order of whatever system his school uses - e.g. SATPIN is commonly first), and look at alphabet books/lists/songs (youtube has lots, of varying quality!) if he doesn't find this too babyish.
Clapping syllables in words (or any other action, like jumping or stomping) is another good exercise for this (weirdly DS has always been able to do this even though he has only recently been able to begin to identify starting sounds and rhymes).
Alphablocks is always fun as well.
Don't worry - sounds like he's doing great exactly where he is. The phonics screening will be based on where NT children are at that age on average. Since you already know he has some difficulties with S&L as well as the visual area it's likely expected that he won't reach those milestones at the same time as others. The screeners are partly to help identify children who haven't yet been identified who might need further support, and partly to check that schools are sticking to the phonics programmes, which do have the most benefit for the most number of children, but might not fit the right pattern for children who are developing at their own pace.