Yes, 100% immersion in a language other than English could well affect all of his verbal-related scores, including, to some extent, phonological awareness. It is well established that young children in fully multilingual environments have slightly delayed language milestones in the short term (and stronger meta-linguistic and executive function skills in the long term).
The tests are normed predominantly on English monolingual North American children (although LEP and multilingual children are in the standardization group, they are usually a small minority), so they should be interpreted with reasonable caution when used on anyone who is not in that category.
