What is the correct sentences in describing my action of showering(washing) my 5 years old son?

1. I help my son to take his shower
2. I shower my son
3. I help my son to shower

i feel the sentence (1) is ideal in ur case.
I agree, out of the 3 #1 is best, but I'm thinking "I help my son with showering" would be better. It might be different in other English speaking countries however.
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For the sake of giving you 3 replies for solidarity, I'll say I agree with the other 2. The first sentence is the best of the 3.
Number 1 is clearly the best of those choices. However, it is still awkward!

"I help my son take his shower"

One does not take one's shower the way one take's one's dog for a walk. They way #1 is phrased is as if you are carrying the shower away not washing in it.

Avoid redundancy of mentioning your son and the posessive "his", you should really say:

"I help my son take a shower" or simply (better, too) "I help my son shower"