If you can, just talk to him. I know it sucks, even being mildly confrontational, but if I were in your spot the convo would go something like this. "Hey, I'm back again. It seems like the last time you replaced the broken spoke(s) they worked very well, but only for a few miles. I know you probably don't rebuild wheels that often, but since it keeps being an issue, do you think it would be possible to completely de-tension the wheel, and true it from slack spokes? I'm concerned that by only replacing the broken spoke and doing a touch up from there, that the others have become randomly tensioned, contributing to the newly broken spokes." I know it sounds overly nice and pandering, but lets be real, no mechanic builds wheels from scratch in general, and the ones that do, with exception of wheel builders, probably don't do it often. This gives them a chance to really fix the problem while saving face, it'll possibly give you a wheel that works, and everyone's mildly happy at the outcome. If that line of communication fails, or the wheel fails afterwards, then it'd be time to switch shops.