How do I teach my child to save up for his own exchange trip programme, when he is only in secondary school?

How about 50-50? Some from his savings, some from you? At least he will get the idea.

Assuming there's 4 months for him to earn the sum. Each months is about $250 or less months.

Suggest you can try all below :

1) Take a 30 to 40% cut from his daily allowances, ask him to learn to meal preps, eat from home somehow some way.

2) Ask him to do housework, siblings ferrying duties and pay him by the hour.

3) Balance treat it as a loan to him and requires him to continue #1 and #2 above to payback.

#1) Sit down with him to understand his cost strucutre and spending and work out a budget plan. This will teach him to learn to budget now rather than in the future to learn the hard way.

#2) Teach him how to clean, arrange things nicely so that he will be independent or even teach him how to cook so that he can cook for the family and learn to self meal preps next time when he goes overseas to study or live alone one day whenever necessary.

#3) Teach him about interest and the rate of compunding (thought you dont' charge him interest) so that he can learn the wonderful world of compunding interest and lean that everything has an opportunity cost so as to learn to choose wisely and manage future choices between needs vs wants.

I think all the above is possible cos i learn how to cook when i was 12 and does cleaning and earn the pocket $ when i was 7 but I got my hand burned on not knowing the rate of compunding when i got my first credit cards and got into huge 10k debts, where i learn the hard way.

So guide them while they are young, they'll thank you for it years later :)

Cheers. Happy Parenting! :)