Personally I would take him out of lessons and try again in 6 months or a year. He's only 3 and has loads of time to learn to swim. Lessons should be fun at this age and if he doesn't want to be there you run the risk of creating an aversion to swimming lessons. I appreciate that you have a pool and therefore it is a safety issue, but you can teach him basic swimming yourself in the summer in your own pool if he would be more receptive to this.