Environment wouldn't "cause" it but perhaps it could influence it? It's hard to tell because stuttering can fluctuate from week to week, day to day, even conversation to conversation. It can be hard to tell but I don't think trying to pinning that down is that important. The more important thing is the feelings of upset and frustration. I would try to let him know stuttering is ok if it happens and his frustration is ok too but you care about what he had to say, no matter how long it takes. And then give him the space to try and figure out how to go at his own pace. If things continue to frustrate and upset him, you could take him to an SLP who is board certified in fluency. However even then finding one who gets stuttering isn't easy. The main thing your kid needs to know is he's ok and stuttering is ok and continues to talk.