Just keep taking him, and he should improve over time. It may speed the process along if you have access to water with a gradual slope to it. I've also seen dogs even up, when they had a bumper in their mouth. They would slap the water going for the retrieve, but swam much smoother once they had it in there mouth.
Lucy was worlds worst when she first started swimming. Tons of splashing with her front feet, but covering very little distance. She turned out to be the fastest swimmer out of the bunch.
My latest Vizsla girl started like that when she fell in a pond. Then she refused to get into the water again - she wanted to but she was afraid. I tried the pool but seen the bottom was not encouraging. Then I took her to San Carlos, MX. and we drove up to the surf. I was still trying to get out of the 4Runner when she crashed onto the surf and started swimming like a pro. I think the buoyancy of salt water and not seen the bottom helped. She now swims in the pool just about everyday but not as good as in the ocean bur she's getting better. Take him to a lake where he can run or walk into it and let her do her thing. Ocean beach better....
As others have said more practice plus something in his mouth. If he doesn't like normal bumpers, try a bird-shaped one like Dokken or Remington make. I think it's a weight/leanness thing too. As he fills out it should get better, but they'll never be as naturally buoyant as labs are.
I love how speedy he is!
Zöldmáli Azúr NA I "Scout" (DOB 4/16/13) Hungarian Wirehaired Vizsla