I'll do my best to offer some general advice, but without specific details on which 'system' your son has been in up to now, it may not be 100% useful. Although having said that, at 12 years old he's still at a relatively early stage of the process, so you have lots of options - which is both good and bad, hence your confusion, for which you have my sympathy.
IB usually refers to the International Baccalaureate diploma programme, which is in available and followed in many countries around the world:
http://en.wikipedia.org/wiki/Internatio ... calaureate
Interestingly, and I didn't realise this until I looked at the above article, there appear to be more schools following the IB curriculum in the USA than in any other country. From the little I know about it, I believe the IB curriculum remains more broadly based in later years than, for example, the A-level system in the UK and Singapore. In recent years UK schools have begun to offer IB as an alternative to GCSE / A-levels and I think it is also an option in Singapore state ('local') schools too. Certainly many of the
International Schools here offer an IB programme, as you have seen.
I wouldn't want to get into a debate about which is 'better' because I think there are advantages and disadvantages in all systems. For students who have a pretty good idea of their general career objectives, GCSE (O-level) / A-level might be preferable in some cases; whereas for those who are still unclear whether to focus on (for example) arts or sciences, the IB route may allow them to keep more options open. The IB programme also has an advantage in being known and accepted around the world.
Returning to your son's options, if he's been in the 'traditional' UK school system during your time in London, he could continue down the GCSE / A-level path at Tanglin Trust School. Or, he could opt for IB at that place or one of several other establishments. Or of course he could also go for the US system - there are a number of schools that follow the American curriculum here. If you plan on eventually returning to the US then that of course would be a factor in the decision making process.
From my own experience some years ago when a possible stint in the US was on the cards, we realised that the 'interface' between the system there and the 'traditional UK' system that our children were following was quite tricky. Don't get me wrong - I thought it was fantastic that there were so many possibilities and options in the American system; the problem was that there were so many and we couldn't quite figure out how to make the 'right decision'. So in the end we decided to go for the 'safe' option and stay here.
Going back to the IB, I would imagine that this would provide a somewhat easier transition to the US system (compared to the UK system, for example), if that is what you actually do in the longer term.
I hope this is of some help; I expect others will chip in with more concrete information. Also, I'd recommend using the search function (use the one in the top right-hand corner above, underneath the 'Profile' button), as I'm sure this subject will have been discussed elsewhere on this forum.
Good luck