もう一回言ってみて！もう一回ママに聞かせて！って英語でなんて言うの？

You can politely ask him to say it again using words such as "may I" or "can you"

A nice way of asking your son to repeat himself.

The intonation and the compliments are important here so as to encourage your son to repeat his rendition. All of these expressions include a congratulatory phrase at the beginning.

Repeat means can you do it again

Yay,that was great!Can you say it again for me? This would be accompanied by clapping of hands and a big smile. yay! = an exclamation to celebrate something good . "That was fantastic!Can you sing/say it again for me please?" You are complimenting him and asking him to repeat the action.

Asking for it to be repeated slowly may help with understanding what was said. These are good ways of asking him to repeat it or tell you the words to what he sang.

"Can you please repeat what you just said/sang?" This is a polite way of asking someone to say what they just said or sang again.