Hubby & I brought our then 8-mths old son to Melbourne for holiday. his first trip. He was jet lagged and by the time he could adjust to the time difference, he got whisked back to SG. Also, the climate difference made him not used to.
My advise would be to go to somewhere with no/not much time difference, and shorter plane flight, and prepare lots of extra clothes and cold clothes for him cos you never know what weather you gonna expect.
Suggestions would be to HK, TW, Perth (only 4hours flight).