How do I explain to my personal trainer that I want to lose belly fat?

Tell him. Or is there more to your question?

Spell it out. The trainer knows what to advise you.

You can’t lose fat from only a specific place, and you lose fat by not eating as much.

For a set of “six-pack abs” you need to get your percentage of fat below 8 %.

“I want to lose belly fat. Please help me achieve this goal.”

Lift your shirt, Point at your belly, and say ” I want to lose this! ”

You won’t lose it quickly.
It will be the last thing to go.
But grab a handful of it, point to it and say “I want this lard gone!!”
That will be a sign of your commitment

Just tell him you want to focus on losing your stomach