My dog can't swim.....?

I have been wanting to place my American Pit Bull Terrier in to 'dock diving' competitions (Like my other three dogs are) as he can pivot himself quite a distance in to our pool, and he absolutely loves water. My kids would also love for him to be able to swim in the pool with them. The problem is that he can't swim - He tries his hardest but he still sinks. My APBT is very top heavy so as soon as he hits the water his rear lifts straight up in the air and as you can guess his upper body (including his head) goes under the water, and i have to 'rescue' him as i am 99% sure he will drowned. His legs will be moving a mile a minute (He knows HOW to swim, he just can't stay afloat). This has been going on for two summers in a row. Is there any way to fix this or should i just keep him out of the water for good? For the last two weeks i have been keeping him out of the water but he expresses his unhappiness vocally as he has to sit back and watch my three other dogs & my children swim.