Yes Tenma was too cruel to Astro. He had an angel for a son and he wouldn't just love him the way he was. He's a child abusing jerk.
I agree with you also about Astro's body. I'm glad Astro didn't allow Tenma to change him into that. Atlas tried to change Astro to something like that in the 80's series, but Livian warned Astro about his plan.
There's a manga and 60's anime story called Astro Stollen, in which some mad scientists convince Dr. Ochanimizu and Astro to try out an Adult body for Astro. Astro tries it but doesn't like it, then the mad scientists steal his normal body and use it for evil. Astro has to fight his old cute self.

Don't worry Astro gets his old body back in the end.
