I disagree, only because if someone said I can't find gum, or I really want gum, you'd respond: he has gum. .. Then you might go over and ask for a piece of gum. (Or can I have some gum.) It's like sliced bread, you'd say I have bread, or I have cake,.. But then discuss specifics.
In English we don't say it that way. We say "He has gum," "He has chewing gum," "He has a piece of gum," or "He has a piece of chewing gum." If you say he has "a gum" it doesn't sound right and it might make people wonder you're talking about the part of the mouth that teeth are attached to.
In this case it's not lo לא, but lo לו. Like their/there or right/write, they are words that sound the same but mean different things. (What's called a homophone).
In this case, יש לו = he has (yesh = there is+ lo = to him) If you wanted to say there isn't = אין / ein (So it would be a different word). Or "he doesn't have" = ein lo = אין לו.
This is in the tips and notes. Do you need help finding them?
(*Learning Hebrew too, but a bit a head of you.)