When you took him shopping, did he show or tell you what he bought? Did you ask him, either "So what did you get for Dad/This One/That One/Other One," or "Did you get something for Dad/et al?"
Not saying you *should* have done this, just wondering how closely you and DH are monitoring his shopping. If you aren't, then it's easy for something to slip through the cracks (yes, even a gift for Mom).
I remember being that age, wanting to be grown-up and choose and buy my own gifts but having limited funds and having to rely on adults for transportation. I would have resented the "what did you get for Whozis" and "did you remember Whozis" questions, because even if I had to be driven to the mall like a kid, I wanted to be treated like an adult when it came to the actual shopping, and not grilled on what I bought/whom I bought for.
Maybe he forgot and was hoping you wouldn't notice. Maybe he changed his mind about his choice and was planning to get you something else. Maybe he was going to make you something and ran out of time. I would give him a few more days and see what happens. Then maybe say something to DH, but not DS, because it *is* rude to demand a gift, but it's not rude to vent one's disappointment to a loved one.