If someone is in his late forties, does it mean that he's about 48, 49 years old?
Yes. Or more generally, over about 46.
If someone is about 42, 43 years old, how do we say it? in his ? forties
... in his early forties.
You can also say someone is in their mid-forties. ~43 - 47.
Naturally, you don't know their age, so the age ranges are approximate.