JMSMommy: I, too, would recommend waiting a while and agree that when you do start, he may end up starting at a higher level.
Are you still doing a phonics program? My DS always tests at least 2 grades higher on language, vocabulary, and reading (sometimes up to 4 grades higher - scary), BUT we continued with Phonics until he completed the 3rd grade level. It was relatively easy, but he enjoyed having an "easy" subject, and it did introduce grammar and spelling. We used Modern Curriculum Press and I totally recommend it. It's a workbook type curriculum, which isn't as intimidating as Spelling Power, and it goes over phonics rules in gradual steps.
If you don't have a phonics program, I'd recommend continuing with one which incorporates spelling (and grammar) and hold off on Spelling Power.