THRASS is a still a phonetic approach to learning, it just incorporates all of the possible sound/letter combinations and has a stronger focus on spelling than just learning sounds like Jolly Phonics does. A bit hard for prep though IMO, especially considering that you can have up to 5 different letter combinations to get a particular sound in some instances. The problem with THRASS - and why we do THRASS at my school with year 1-6, not kinder - is that there's not a lot of focus on single sounds as a separate thing. Jolly Phonics explicitly teaches each sound, which then progresses to diagraphs and blends etc. He should be fine with it, because I imagine that if they are using THRASS at a prep level then they are going to know that some students will still have age-appropriate issues with sounds and will be able to help those kids master it. I think that if he's OK with understanding it and it's just how he says it, then there shouldn't really be a problem.