Russian tortoise doesn't like soaking?

by Victor
(USA)

I adopted a Russian tortoise a while ago, and he seems to be doing well, but everytime I soak him he walks backwards for a little bit and then after a minute tries to climb out. I make sure his water is lukewarm, and not to high (the water meets a little above the bottom of his shell). I'm getting worried because I don't want him to get dehydrated and sick, but I'm not sure what I'm doing wrong... can anyone help me?