M.M.
Hi, my kids are now 15 and 13 yrs. old. I took them for swimming lessons when they were about 3 & 5 for the first time......my younger son didn't want to go into the water at all.
I didn't force him to go in and wouldn't want the instructor to force him either. After that I signed my kids up for semi private lessons, I think that worked out better for them.
My little one got better and they both learned the basics. Now they both love swimming, and are pretty good swimmers. We just got a membership at lifetime fitness and they love going to the pool there.
So I think all your son needs is more time to get used to the lessons. Don't force him to do anything, cause that will only make him more scared of swimming. Maybe you should give him another year before signing him up again for lessons, or try private or semi private lessons for your kids? The semi private lessons worked best for my kids, maybe they would work for yours too.
Good luck!!! hope this helps.