A child has to want to learn, to get any benefit from tutoring. IF they are going because their parents want to and not them, then it is a waste of money.
If your DS is being so resistent at home, then this is probably not the best time to use a tutor.
Does he respond to computer games? There are loads and loads of spelling websites - some you can put the words in, and they make learning fun.
Sentences can be related to topics of interest, whether its star wars, football or whatever they are interested in, the teacher wants to see if they understand the word.
Getting a child to do homework when they are not in the mood doesnt work either. This makes it a chore and not enjoyable and if they dont enjoy it they wont retain it.
If he is struggling to make up sentences, may be the words are too hard, for now. It might be that writing the sentences is too hard.
Other suggestions perhaps are, doing spelling homework with a group of friends, or make it competitive, or link it into something he enjoys doing - beating his mum, timing, and by making a game of it. You could both write sentences and decide which is best (his of course) then this should improve his confidence.
We did computerised tutoring for a term, similar to Kip McGrath, and yes it improved his confidence, but in reality it was a complete waste of money. There was supposed to be one to one teaching, but the tutor had four kids, so they only got one to one when they asked for help. If they didnt ask, they didnt get help at all and it was extremely expensive.
Our school is dead against outside tutors, mainly because of the cost factor and the fact that they havent made that much of a difference to the majority of kids. Sometimes a term can be the trigger to turn the lights back on, but its making sure you get the right package.
Me personally, we now stick to the online tutoring programmes, cheaper and just as effective.