I'm planning a trip to Singapore with my family.

My son has 2 passports, one Canadian and one Thai.

The Thai passport has only 2 1/2 months left, so he will use the Canadian passport to enter/leave Singapore.

But will he have any problems leaviong/entering Thailand using the Thai passport ?

I think he shouldn't, but TIT,

No problem leaving and entering on a Thai passport and leaving and entering another country on another passport, as long as you do it by air (as your son will). Only at land borders it would not be possible.

No problem leaving and entering on a Thai passport and leaving and entering another country on another passport, as long as you do it by air (as your son will). Only at land borders it would not be possible.

I know there will be no problem in Singapore - the airline specifically asked for passport details and said if passport validity is less than 6 months boarding would be denied.

But what about coming back to Thailand using passport valid less than 6 months ? I know Thai immigration sometimes applies this rule to foreigners, but could they do it for a Thai on Thai passport valid less than 6 months. ?

It would be strange if they did, but I've seen crazier interpretations of "rules" here before.

No problem at all entering Thailand with a Thai passport with remaining validity for only two months or even with an expired Thai passport.