MaggieW
I've never done quite so far but did do Hong Kong to UK and back when my dd was 4 1/2 mths old (13 hours) In our case it was fine (we are about to do it again now she is 1 and I'm nervous again!)
In our case, it was a night flight (which I think the first part of your flight will be too?). I did her normal routine, naps etc at home the day we flew, even doing bedtime routine and putting her to bed for a couple of hours before we went to the airport.
When we got on flight, she cried for first hour or so, (noise of engines as we took off and bright lights while they served food etc.) She was so tired... but as soon as lights went out, she slept for about 7 hours. (bar when we hit turbulence and you have to get them out of the skycot....and strap them to you again...). She woke up about half way through the flight (regular wake up time.) and fed her morning bottle, then she was awake for a couple of hours. I was lucky and travelling with dh, who walked her round the plan a bit. We flew Virgin and the crew were fab - passed her around and cooed at her at the back of the plane for a bit and chatted to my dh.
Then she napped for a bit before we landed. When we arrived it was 5.30 am UK time and we took onward flight a couple of hours later (just 1 hour to Scotland). I then started to largely follow her normal routine, but had made a note of HK times for bottles and the equivalent UK time so that I would give her enough at each feed, but tried to steer her as much as poss into "normal" routine. I let her sleep a bit more than usual, but not too much more and took her out for walk in sunshine etc. Then did bedtime routine as normal and into her cot.
Re. getting over jet lag, she was fine in UK (woke up first night for an hour or so but after that absolutely fine) but coming back to HK we had a couple of nights where she woke up screaming and hungry in the night. Although she had been sleeping 7pm - 7am, I went back to waking her at 10.30pm before we went to bed on the second or maybe third night, did that for a few days, by which time she was fine. During the day, I strictly followed her normal routine and didn't let her sleep too much.
Other advice - we were told to give her a bottle of take off and landing to help her ears adjust. Take spare clothes for you and baby .... I've heard horror stories of Mum's needing to fly 24 hours in milk spewed clothes! (Ugh!) Our dd sleeps in a sleeping bag, so we took that for the sky cot (also take a sheet or baby blanket to line the sky cot because from memory they gave us the blanket and pillow you get as an adult.)
Ask for an ailse seat so you can get up and walk around if necessary.
Take extra bottles and nappies (more than you need in case you are delayed.)
And calpol in case he gets v. distressed.
Other than that, tell the airline (before you arrive at the airport) that you are travelling solo with young baby, and they should give you extra help with luggage etc. They will let you take your buggy right up to plane door. (V. important that you tell them you want it again when you arrive in Singapore and you want it as soon as you get off the plane. ie at the door.) If you use a baby bjorn have that too, then you can put the baby in the baby bjorn and all the heaps of gear you will be carrying in the buggy...
Can't think of anything else at this moment...probably too much anyway!
Good luck - I'm sure it will be fine. Fortunately, at 5 mths they still sleep quite a lot so the time will pass better. Your baby is likely to adjust to the jet lag better than you!