I think that imaginative homeworks are good for year 2 children. Rather doing it for him, I suggest you break the homework down into managable chunks.
For example with the following homework: "thinking of monsters nice and bad, and describing each of them"
Have two pieces of A4 and in the middle of one piece of paper write "nice monster", this technique is a spidergram . Ask your son to verbally describe a nice monster and write his ideas down on the sheet. You could get a series of lines (legs of the spider) coming off the spider with the words at the end of the line. If he likes drawing then ask him to draw a picture of his monster in his book.
When you have done this, sit your son down and set a timer for 20 minutes and ask your son to write about the monsters. (Make sure he has been to toilet and is not excessively hungry or thirsty first.)
Don't ask for 100% perfection. The idea is that its his work. Spelling mistakes, forgetting about finger spaces, punctuation, capital letters often happen at this age. Praise him, if he makes a serious attempt, even if it is not as neat and beautiful as when you sit there breathing down his neck.
If he refuses to comply with sitting at a table for 20 minutes then I suggest that you punish him like any other form of disobedience. With my son who is also in year 2 I ban him from watching TV or playing on the computer for the rest of the day.
We are lucky in that my son has a week to do his homework. In the situation of my son's homework not getting done on a particular day, I would try again on a different day.