If the school is teaching synthetic phonics, he would have been taught how to spell words with these phonemes in Reception. However, that doesn't mean he learned how to do it. Sadly, the synthetic phonics scemes go at a fast pace - fine if you are the kind of child who grasps this easily, but not fine for many children and especially not young boys.
If you find that he is struggling to hear and reproduce the sounds correctly, then I would advise you to speak to the class teacher / SENCO immediately and ask what they are going to do to ensure that he gets taught the next steps properly.
Does he know all his initial sounds - s, a, t,n, i, p, etc? Can he read and spell CVC words like cat, big, sun, correctly? Is he confident with short vowel sounds - like the ones in the previous words - say if you said net, would he spell it with an e or an i? If he can do all that, then he needs to be re-taught, with loads and loads of practice, the long vowelsounds like ai, ee, igh, oa. There are loads of free games on the internet to help with practice, as well as the schemes mentioned above.