Hey Claw, followed your link from the other thread.
The two subtests are looking at different things, it would be quite possible to score 90th centile in one and 0.4 in the other.
The second CELF subtest and the Renfrew word finding tests are looking at similar things,both to do with vocabulary.
The CELF one looks at the child's ability to make links between words.
When you learn a new word, you store it in lots of little 'pockets' in the brain...so for example, the word apple - as well as knowing the object, it gets stored in the 'fruit' pocket, the 'green' pocket, 'grows on a tree', 'begins with letter a' and so on.
The subtest checks that those links are there by giving 3 words and the child picking the two that go together - that might be that they are in the same category, or a related like knife/fork, day/night.
The Renfrew Word Finding Test looks at vocabulary knowledge in a more simple way - child gets shown picture, names picture.
The results of those subtests indicate your DS has lots of difficulties around vocabulary, categorization and making links between words.
The first subtest is not looking at vocabulary per se (obviously the child needs to understand the words used) but your DS only needs to access the vocabulary in a simple way. To understand the sentence 'the dog ran down the road' he's not having to think about 'dog = animal, furry, pet, doesn't like cats...'; he only needs to access 'dog' in one vocabulary 'pocket'.
The subtest is looking more at the ability to 'read between the lines'. For example if a teacher said 'it's very noisy in here', the implication is that she wants the children to be quiet.
It's quite hard to explain, but I hope this helps a bit! Obviously I don't know your DS, so I don't want to speculate on what impact the difficulties with vocab will have on him specifically, I would say definitely give your SLT a ring and ask for further explanation!
I'll keep an eye on this in case I can help any further x