Wow - how great that your Ds is so enthusiastic about learning to read already.
Try shared reading - read books to him that are harder than he could manage alone but let him read occasional words that he can manage with his current knowledge. Let him choose his own books from the library/bookshelf (within reason - don't start on War and Peace!) and just enjoy them together. This is probably the most important single thing you can do.
You could look at other phonic reading books that are on the same level he can already manage (Songbirds Phonics? Read Write inc?). Many of the bargain books companies ( Red House and Book People) sell massively reduced sets of these.
Try some websites - there's lots out there for free with simple phonics activities for a 4 year old. I like Starfall if you don't mind the North American Accent. This might help you to start to tackle some of the more complex phoneme/grapheme correspondences.
Software: I like Nessy products and you can get their Hairy Phonics program for less than £20 as a download (and there are free bits to try on the website to see if it suits). here Again this may give you confidence to start to teach other graphemes using the structure of the program if you wanted to.
If you pm me an email address I will send you a copy of a booklet I have produced suggesting fun games and activities that parents can do to help children learn phonics and suggestions of websites that might be useful
Once he is more settled in at nursery they will (hopefully) realise that he knows most of the basic single letter/sound correspondences. If he is showing signs of getting bored/turned off by things being too easy then you could ask them if it would be possible for them to differentiate some activities at a more complex level for your DS and any others that are ready for it (but do it in a friendly, non-pushy way!!)
Think about contacting a local tutor with experience in teaching student through phonics. Before anyone shoots me, I'm not suggesting that a 4 year old has formal tutoring every week - but - if he is showing frustration at not being able to progress further and you are not sure how to help, you may find a tutor who is willing to offer you advice on what to do next and maybe just run a couple of sessions with you to show you the next phonic stages. I (as a tutor) would certainly be willing to do this as long as I was sure the parent wasn't pushing the child faster than they should be going. I'd probably want one session with your ds to assess where he was at and see if I though he was ready for more, but then I would be willing to work with the parent to guide them on how to do it!