I disagree with it is impossible, as I know people it is their reality to have ND and PG kids. OTH, My DD5 and DS12 test 2 points apart on FSIQ. They have very different strengths. IMHO, When you have a little guy as off the charts as yours, the other kids in the family may not look as strong, but in all reality are just displaying their abilities in a different manner. I would not worry about testing a 2.5 yr old anytime soon, but I would consider it in a couple of years. I would be careful not to dismiss things because they are different then your older child's development. My DD looks very different in development compared to her brother. DS12 was an intense, driven, loud, and proud display of his abilities at 5. DD5 will not show you her ability to do something until she is confident she can do it. She spent at least 6 months telling me should could not read despite a lot of evidence to the contrary. She went from not rolling over or sitting up at 6 months, I thought she might have delays because it was so late, to walking before she was 11 months old. She just seems to wait until she is good and ready to show her stuff.
In re to the LM showing PGness that is hidden on more modern tests, I'm still not convinced, honestly. The earlier article seemed to indicate that it was creating scores not simply revealing them. I'd like to see it renormed with a modern group of kids including average kids to be convinced, but I know that isn't going to happen. It isn't relevant for your kiddo, though, anyway since he didn't test MG on the WPPSI.
I really dislike the use of SBLM. I think in this case it doesn't change the outcome as your DS is clearly consistently PG in other tests. I think too many parents could get an inflated score on an outdated test and put their children in situations way beyond their ability or have expectations that are unrealistic. IQ tests can have strange and intense reactions for parents and the bell can't be unrung on for them. There is a reason no one (at least that I'm aware of) will accept the score as legitimate for the entrance into a program. My tester mentioned that my son could test on it for giggles, but it's a useless number in terms of information to be gained from it
