From swallowing semen? No, he can't. That covers the question of whether or not he can get HIV after swallowing infected semen--as in after it passes down his throat, can it infect him. No, it can't. Not unless his esophagus has a cut in it. Once it's in the stomach, it's not a problem. So the answer to his initial question was, in fact, "No."
And no one told the poster that "You cannot get HIV from oral sex," which is what Madman accused posters of doing. You can get HIV from oral sex. But barring a ripped up throat and esophagus and a torn up stomach, no, you can't get HIV from the act of swallowing. It passes too quickly to transfer through mucus membranes in the throat and the rest of the esophagal tissues are not mucus membrane, and the pH level of the stomach is too hostile for the virus to survive.
The region to beware infection is in the mouth, specifically the gums, which is what I explained after the hesitant statement "...No." (Ellipses mean something, you know.)
And as if the rest of my post didn't explain where the danger was instead of some misleading 'absolutely not' statement. Let's read posts, people.
But you are correct to warn him of other STDs that he can get from oral sex, not requiring contact with semen.