Hi bunnieboo. I flew to and from Singapore with young children quite a few times. It was OK, the worst for me was that I couldn't get to sleep myself, but they were fine. I would say definitely get an overnight flight, maybe leaving early evening, then your DS can have some food, watch a cartoon, change nappy etc then you can settle him down to sleep. If I remember right, you don't have to pay for an extra seat for under twos, and you can book yourself a seat that has a sky cot - so it's a seat at the front of a section that has a cot you can attach to the partition in front of your seat. If he's like mine and doesn't fall asleep easily then it might help to have a muslin or other cloth that you can drape over it to block out some of the bright light.
Another good idea for hours when he's awake is to bring some small, newly bought, toys / little books / colouring stuff (you could even wrap them up), so that if he has a fractious moment you can say, look what we've got here, a new plastic toy car! Also some snacks.
Jet lag does make it hard when you arrive. The best thing is to try to get into a pattern of eating and sleeping straight away to match your destination - so if you arrive at say midday Singapore time then try to stay awake till bedtime (or let him have only a short nap). Apparently the time we eat helps set our body clock, too, as does being outdoors in daylight. If your family live in a condo with a pool, then no doubt he'll love a splash around, and it'll keep him awake.
Also, you could post this in the Living Overseas section, lots of experienced travellers on there.