If you think he could get 2a without too much pushing, then there's no harm in asking the teacher exactly what he'd needs to do to get there.
Tbh, (and I hope you dont think im rude for saying,) if I was in the same position as you & was going to do some extra work with my dc, I'd want to focus on the area he was weakest on & try & bring that level up. If he is a strong reader there's a chance that he might do better than the teacher is predicting anyway (ds went up 2 sub levels in the last term of yr 2 in one section). Of course theres nothing wrong with him getting a 2c in writing & like you said he is one of the younger ones in the year, but it might stand him in better sted for ks2 if he was at the expected level in all three subject areas. Maybe you could ask the teacher which she'd recommend doing if you were to do a bit of work with him at home?
There is always another possibility, you could consider combining the two by getting him to write a bit about books he's read, (why he liked it, his favourite bit, how the main character was feeling at certain point in the story, explain what happened in his own words etc, etc,) or reading half a story with him & asking him to write his own version of what happens next.
You know your own dc & whether he's likely to do/benefit from something like that, my own ds would never voluntarily do work out of school, while my dd loves to!