Prepaid card might not be a bad idea at all.
How long is your son travelling for, short term of a few months or longer term and roughly what locations. What age and is he good with money. If longer term travel I think a credit card would be good to have.
I traveled eastern Europe, Asia, Aus and New Zealand (All apart from Aus was mostly from my Irish account). A credit card will be more widely accepted than debit card. Depending on the debit card, fees for non Euro may be big.
Example BOI debit non Euro fees '3.5% of transaction value. Minimum €3.17, Maximum €11.43 per transaction'
If he was to withdraw a few thousand over a few months he could pay, 100, 200, 300 or more in fees.
For me I did something that lots will not recommend and had my TSB visa ice card in credit, topped it up regularly by small amounts online. As far as i know the currency conversion fee with this card is still 1.75% (no cash advance fee if in credit). Correct me if I am wrong but the protection on money in credit on a credit cards is no worse than on money on a debit card. Over the time I was travelling and using my card abroad (about 2 year in about 20 country) this probably saved me 300 Euro or more compared to my BOI account.
Banks may have better fx rates inside than the ATM's as well, I am not sure on this but withdrew sometimes in branch.
If your son is not good with money and would likely get charged late payment fees and interest, a prepaid credit card may be better.
If travelling south east Asia a few hundred dollars hidden away separately as backup is a good option as accepted a lot. In some countries (Laos I think) max limit on ATM withdrawal was low (equivalent of about 100 dollar) with extra atm fees involved.
If travelling long term I think 2 cards from 2 separate accounts is best as backup. If he only has one card western union is available nearly everywhere as backup
Notify bank before travelling so card doesn't get blocked.