I grew up until age 6 or so on the beaches in Honolulu. The zero-entry and mild surf was something I enjoyed, but I never learned to actually swim out there. We moved to the Mainland (Oregon) when I was six and then I was taken to the Y for swim lessons.
This was the worst thing in the world, to me. No low place to just sit in the water. I was a short kid even then, so even the 4' level was a bit much. (I had also been traumatized much earlier-- one of my earliest memories is my mother being baptized and thinking she was being drowned.) I *hated* the swim lessons, cried, would cling to the ladder, would barely blow bubbles.
Then at 8 or so, I was thrown into a pool with a floaty ring around my middle--and ended up upside down, nearly drowning.
I swim now and love it. I never took a class. It just took me years to teach myself to dog-paddle (at hotel pools, as I got much older), it took me forever to relax enough to learn to float. But I did it right before I went into the Navy. A friend had advised me that it would be better for me to learn before boot camp, or I'd be in 'remedial' swim lessons. That's all it took.
But I still swim with my face out of the water, like a dog. Not cool, but it works.
Give your son time, do as others have suggested: play with him in the water, where he's comfortable. Once my folks stopped fighting me on not wanting to go into the water, it made things much easier. Seeing siblings having fun in the hotel pools made it more enticing. Some things we just need to figure out on our own. My son is 5 and only just started getting comfortable with getting his hair washed (just nervous laughing now, not screaming each and every time anymore). Being fun, silly, confident that he'll do well and being patient that he'll get there on his own will help. If you have confidence that he'll 'get it' eventually, so will he. When you can see that spark of confidence, get private swim lessons as Cheryl C suggested.. there was nothing worse than seeing other kids looking on in derision, and kids are often grouped by ability, not age. Being in a group of 4s and 5s when you are 7 is a blow, to be sure.
Sorry so long--- just to tell you-- there is hope, even for the most hopeless cases like I was.:)