We started taking Nisha to the lake at around 3 months (we had a baby pool set up at home too). At first she just waded around where she could reach the bottom. We
never pressured her to go in; I think this is a bad idea. You should let them go in at their own time. Well, one day we were at the park and a stranger picked up her ball and threw it in the water for her (lots of people throw balls for "stranger" dogs). To our amazement she leaped in and went for it
. We were so proud!!! This happened when she was about six months old. She has been a fish ever since!!
So...my suggestion; set up a "baby pool" in the yard. Get him used to playing in the water in a controlled environment. Then take him to a lake that has a
gradual incline (beach type); this way he can go in as far as he feels comfortable. Eventually he may decide to "go all the way". I have heard it helps if you swim with them.
I have also heard that some Rotties don't like water.