赤ちゃん (aka-chan) is a familiar derivative of 赤ん坊 (akanbou). 赤ちゃん (aka-chan) is only used for one's own child.
According this dictionnary entry it is said that 赤ん坊 is a fetus, a newborn or an infant. An infant being a child less than 18 months old or that cannot not walk nor talk. It is fairly close to the meaning of the English word "baby".
”Akachan” is ”baby.” The meaning of it has not far from born and depending mother. If he walk stable or talk, or you talk with one who thinks he is not a baby, you cannot use it.
On the other hand, you can use ”musuko” even if he is a baby. But, ”-chan” is not suited. If you talk about other’s son or daughter, you can use ”musuko-san” or ”musume-san, ” or ...
First off, I think you got the actors the wrong way round.
I ended up getting fat again.
That's because you eat nothing but sweets.
Can you remove からだ? Grammatically you can, but it wouldn't sound natural in the same way that this English exchange would sound slightly awkward:
A) I ended up getting fat again.