Set up Apple Pay says If you're under the age of 13, Apple Pay isn't available and you can't add a card in the Wallet app. Age varies by country or region.
However, like you, I can't find a comment about which countries this might be. Perhaps you could ask Apple Support.
Be sure also that the card you are attempting to add to his wallet is a local card and not from another territory. It must be from one of the following Singapore banks, according to the list in Apple Pay participating banks and card issuers in Asia-Pacific
- American Express*
- Citibank (Mastercard and Visa credit cards, Mastercard debit cards)
- DBS (Mastercard and Visa credit and debit cards)
- HSBC (Mastercard and Visa credit cards, Visa debit cards)
- OCBC (Mastercard and Visa credit and debit cards)
- POSB (Mastercard credit and debit cards)
- Singtel (Visa Prepaid Card)
- Standard Chartered Bank (Mastercard credit and debit cards, Visa credit cards)
- United Overseas Bank (Mastercard and Visa credit and debit cards)
- Wise
*Bank-issued American Express cards aren't supported.