Having a dad that used to write speeches for a living helped me.
But seriously, he should visit the reception venue, with someone else who should stand at the back of the room to see if he can be heard, without shouting.
If you have never made a speech, the tendency is to shout instead of projecting your voice, especially if you are nervous. The best thing is to use a microphone,like I did.
Don't bother making jokes and write down your speech, there is nothing wrong with reading it off a piece of paper.