Yes there is no magic in learning to read, it is just a process which some people pick up fairly quickly and some need to go through in more detail.
Learning to read is a combination of (i) learning to decode words into sounds (phonics) and (ii) learning the common words which don't decode properly by sight eg who and said.
You need to do a bit of both in small chunks. The great thing about phonics is that it is very logical. English has 26 letters but 44 sounds. Synthetic phonics teaches you to learn the sounds so that you can split the word into its component sounds. Some are just letters eg c-a-t and some are groups of letters eg igh. Once you have learned these they enable you to spell out any decodable words and once children realise this that is normally accompanied by a surge of confidence and progress (for example what does d-oe say? OK then what does r-oe say? What about h-oe?) ("say" is not the approved terminology but it is the one which seems to make most sense to learners IME).
This sounds a lot more daunting than it is and if he is already reading at Year 3 level he probably has some basics, but is relying a lot on sight memory. I have taught a lot of children and adults to read, some with additional needs, and whilst some learn slower than others, they all learn.
Despite his age don't be afraid to go back to basics. Often the problem is that children have learned enough words by sight to pretend they are reading but they can't apply that knowledge in a different context. So you need to work out how much he really does know.
I'd say look for a good synthetic phonics programme and use that. Personally I like Ruth Miskin who has a great programme which includes charts of the sounds and little booklets where you read stories which just use the phonic sounds in the order you learn them, but it is quite expensive to buy everything.
It is perfectly possible to make your own charts, stories and flashcards if you read up on the subject carefully.
The key is repetition too. In one session I start with a chart, then we go through a short story, then flash cards. Try and find stories with a humorous element if possible. Humour, praise and bribes go a long way :).