Hi, There is some research indicating that those who are bilingual are more susceptible to stuttering, so there may be a similar thing happening here. So I suggest sticking with one language for him for the time being and seeing how that goes. Additionally, there is some research looking at how stuttering develops and there are some theories that children don't necessarily feel rewarded when they speak so I suggest rewarding your child with praise when they are speaking fluently. The main mover may be sticking to one language, but if it continues to be a concern I suggest getting it evaluated.