"From now on, he is going to become" doesn't make sense. "Is going to become" implies something that will happen in the future, so "from now on" doesn't fit with that.
Agree with ncp. If you say "from now on" it is natural to say "he will be famous" -- "become" is awkward in that context... Unless you said something like "he will become more and more famous."
I agree. Becoming something is not a continuous action. It should be either "Now he will become famous" or "From now on he will be famous".
I agree with the others: it should either be: From now on, he'll be famous OR: Now he'll become famous. The translation given here is redundant and sounds unnatural.
This is a French sentence, so both going to and will should be accepted in an English translation.
In other words, désormais just reinforces the concept of immediacy/proximity without emphasising the difference between stative or action verbs.
I agree, so I wrote "Désormais, il va devenir célèbre." = Now, he will become famous" but the answer was said to be wrong.