I don't let my daughter (who is 5) use the rotary cutter. I can't imagine her being ready at 8 either, given how sharp and really dangerous they are (and how very distractable she is). I don't cut everything for her though - I'm just teaching her the "old fashioned" way, of tracing templates and using scissors. Actually, at this point I trace and she cuts, but as her skills improve she'll get to do it all.
But I think you are the best judge of his particular readiness - you know what he's likely to do or not do.