Hi - retired TA here -
I can understand this is frustrating and confusing, probrbly for both of you, if your hearing isn't 100%, and his pronunciation is also unreliable at times.
FIRSTLY - 'choose-ed' is NOT a problem, because he is applying his knowledge of 'past tense' words (though, obviously not knowing that's what it is) according to how the majority of words do end in '-ed'. So a child will also say 'run-ed', until he is mature enough to learn 'ran'; so don't worry about that aspect for now.
SECONDLY - the 'ran - van' problem. The sound 'r' should not be 'ruh' strictly speaking, but a purer 'rrr' (which, I agree unless you are Scottish, is a difficult sound!) Similarly, 'm' is not 'muh' but 'mmmm', and 's' is not 'suh' but 'ssss'. I think that Cbeebie 'Alphabats' has correct versions of Phonic sounds, and there are also web sites that do.
But, rather than CORRECT him when he says 'van', turn it around and ASK him what sound 'van' starts with. So, if you can avoid correcting him, but instead give him clues to lead him towards the correct answer, is probably better.
THIRDLY - An inexpensive and easy to use book, that can encourage children with reading, spelling and writing, and really help them to understand Phonics, is reviewed in the MN Book Reviews section. Just search ‘Phonics’.
================================================================
FOURTHLY - When I worked with less able Yr2 children, who were finding learning to read particularly difficult, we often used a SoundWorks kit, which consisted of a set of wooden letter blocks, which the child used to build simple words.? The theory was that, for some children, it is easier to SPELL words than READ them, which is a later stage.
It started with three-letter words, with a vowel in the middle - "a" glued onto a board.
The child then looked at the individual letter blocks, and was asked to make the word "c a t". Then he was asked, how do we change "c a t" into "h a t", which letter do we need to change? Then change "hat" into "ham" (with an emphasis on the "mmmm" sound).
Work slowly, and pronounce the sounds accurately and clearly. This approach was used with our Yr2 children who had been unable to make progress with more conventional methods of learning to read. It is rather time-consuming, and ideally needs resources to be made, but it does work very well.
So, if you can find or make suitable letters, and make a card with "a" glued in the middle, your child may enjoy building the words. Use letters that are occurring in words in the books he is bringing home, and then go on to make cards for the other vowels if it seems to work with "a".
=================================================================
I am sorry this has turned into a lengthy answer, but just take from it any bits that seem relevant to your situation.
Good luck, and come back if I can help any more.