You've gotten a lot of good advise here. There is another option too. Forget about the water for a while. Keep working on his land retrieves, and his yard work. Keep the training fun so the dog is jazzed up everytime you train. Then in a few months, casually take him to the water and see what happens. If you want to wait a bit longer, wait until the water is warm enough for you to swim in it too. I'll bet if you swim he'll follow you. For now, however, don't pressure the dog to enter the water. this will only worsen the anxiety.

As was stated in other posts; don't throw your dog into the water to make him swim. This will only make the dog not trust you when water is nearby.

Also, have you cosidered that maybe the dog won't enter the river because at this time of the year the current is stronger? Just a thought.

Good luck