my 18mo, son born in thailand with thai passport now lives in australia with aussie passport will be traveling back to thailand for 4 months with his mother who has thai only passport. will he need a visa. i think he has to leave and enter australia on his aussie passport.

Leave Aus with Aussie passport and enter Thailand with Thai Passport.

As above, he leaves Australia on the Australian passport and enters eand leaves Thailand on the Thai passport. (Check with Australian immigration if that is alright this first time, as he entered on a Thai passport. Maybe they want him to leave on the Thai passport and when he comes back enter on the Australian passport.)

At check-in with the airline simply show both passports, that way they know no visa is required.