I think it's not abnormal for a highly gifted child to have lower working memory and/or processing speed. Those are on the low side, but the speed might have been just because your son was taking his time to avoid mistakes. Is he the meticulous type, or do you believe he was being extra careful?
I think real experts will happen along and give their opinion, but I wouldn't hold off on a grade skip because of these scores alone. The working memory and processing speed are still within a normal range, even if those scores are accurate.
Think of it this way: would a child, in the grade to which your son would be skipped, be unable to perform if they had a similar working memory and processing speed? Your son's in the same boat, except that he's brilliant. In addition he is probably at least at the proper grade level academically to support the skip right now.
