My 9 year old boy and I are building a 8 1/2 foot 5 weight RX6 (4pc) rod I gave him for Christmas.

I need to get the best line to match the rod for him as he would be considered a beginner. He has been fly casting with cheap rods and lines for the last two years. I was considering the scientific angler 6wt head start line. Does anyone else have any suggestions or experience with this line/rod combo?