Brief history: son was an identical twin but they developed severe twin to twin transfusion syndrome and I had to have emergency surgery to separate the interconnecting vessels in the placenta. Twin 1 was the donor twin so tiny and basically shrink wrapped in his own amniotic sac, twin 2 was about twice the size and on pre-surgery scans was basically bombing around in his amniotic fluid swimming pool. The general consensus at the time of surgery was that T1 had poor prospects but T2 would be fine. In fact T2 died within a week but T1 made it through and started to grow again and was born at 32 weeks weighing 2lb 12oz. Just to make life that little bit more heart stopping, he then got incredibly sick with an infection, started to recover, had a TPN line put in which migrated to his lung and nearly drowned in the feed until they realised what was going on. He eventually came home on a smidge of oxygen which he came off pretty quickly, and although he has managed to meet every milestone at just about the last 'normal' point, is basically now a happy, healthy, talkative and bloody cheeky 3yo.
We looked into antenatal twin death a lot when T2 died (I was resting up in bed for a while and had a lot of time on my hands). A lot of the stories related to people who'd been told in much later life and had apparently ascribed a general sense of 'loss' in their lives to this event that they had never known about. Husband and I are both natural born cynics, and we both felt that actually most people, if told they had a lost twin, would feel some 'loss' about that, just by virtue of the fact that life is sometime a bit isolating, so neither of us were convinced that he will actually ever really remember unless we tell him.
My view was that we should just deal with it at a very young age, but husband felt it might be better never to say. I felt that that was quite wrong as someone else (parents, friends) might let it slip (although in reality we are the only people who ever mention T1, I think everyone else is worried about upsetting us).
When I was in hospital after son was born I had a very brief session with a nurse-counsellor there (it was an excellent hospital medically, but given what we went through, the emotional support side was fucking disgraceful) and she basically said that we really ought to tell him, so we made up our minds that we would.
Naively, given that this was pfb, we thought that we could probably do this at about 2, but he's now well over 3 and he would be confused to shit if I started talking about his non-existent brother and I'm sure he'd look at me with his 'mummy's gone a bit mad' face (mostly used when I tell him it's time for nursery and he's playing with his trains) and inform me that he doesn't have a brother.
So, at the end of that (not very brief) history, how long do we wait to tell him and, if anyone has done it, what do we say that a) doesn't upset him, and b) doesn't make him think that we're going to suddenly produce a brother?