50 mins is a long time for a 6 year old, a really long time and to set a word limit sounds like lazy teaching to me (disclaimer I am a teacher and of year 2/3) It would be better for her to write the title and then some objectives the children must have in it such as at least 10 describing words, 4 facts etc etc, boys (most boys) do much better when there is a clear structure to follow, that they can tick of when they are finished, which is why most flourish in maths it's not always the content, it is because of the clear structure and ability for them to see how to be successful; in maths they know they have to do 25 sums (or whatever) with a title like snails and 50 minutes what's their success criteria, how do they know what the teacher wants, except 100 words....
To improve boys literacy, I used to (at home currently) do things like; get them to draw the story/idea first in cartoons style then add the words, never set a limit of words, allow them to write in colour,pen,felt etc, put on funky music during our writing session and a lava lamp so it felt cool, encourage them to write on smaller paper so they felt like they filled it like on post it's or envelopes, get them to write spy messages to each other, desgin their own comics, commentate on football matches, then write a match report for a current team like today's fa cup, write adverts, etc etc basically short, sharp, interesting tasks, writing a rap, a song, a letter to a pen pal....at home you could do some of these things, have fun/cool writing implements lots of different styl of paper, he could write you or someone in the family a letter etc
I've seen boys achieve a level 3 in year 2 with less than half a page, the marking criteria is not about length; a very well known prep school took a boy on 3 lines in his English 7+....
I'm sure you already encourage lots of reading, but lots of boy friendly books will have a huge affect on writing creatively and if he is having trouble with actual writing/hand writing there is a great boy friendly workbook I will post a link to in a minute....
It is hard at this time of year, but if (without sounding cheeky) your son can ask for guidance on what the teacher is looking for it may help him, make a checklist to work through....
Sorry this was so long :)