So this is a bit off topic, but it seems like a bad idea to not teach both the sound and the name. For one, the letters don't always make the same sound and for another, it seems like it would be confusing later. And having a name and a sound really isn't that confusing. Think about animals. Kids learn early on that animals have both a name and a sound. Seems letters should be treated the same. I started talking about letters with both the name and sound from very early on and it was never confusing for my daughter who is now nearly 4 and knows many letter names and letter sounds (I don't exactly quiz her so I don't know how many she knows). And she also enjoys spelling things with me (I sound it out phoneme by phoneme and she says the letter that goes with each phoneme) and I'll make it a point to point out that sometimes letters say different sounds and sometimes you can't hear them at all.
This is exactly right. It's one of the biggest problems we face when parents start teaching their kids phonics or the alphabet. I'm a teacher and what you describe is pretty much how we initially teach phonics. We introduce letter sounds 'a', 'o' etc then very quickly move onto sound rules 'ae', 'oo'. Another frustration is that some parents also incorrectly sound out letters by not using pure sounds.
My advice to the Op would be chat to your son's teacher if you have any concerns.