Goodness that's quite a discrepancy! You don't say how old he is, the younger children are the more variable their results can be on any test (bad day, didn't like tester, etc). But also, the WPPSI4 is very new and yes it is producing lower results than some might expect, I have no idea about the slossen, but if its gettin long in the tooth then yes there could be a serious gap in the morning between the two tests, which added to a good day/bad day scenario could exain your problem.
I really feel for you, I have an extremely difficult to assess and changeable 2e child, it's not easy to find answers tha seem accurate and reliable.