Keeping him out for the entire week will result in a huge pile of work -- which he will need to write to complete.
If you are going to do this, then get his work from school and have it do it at home this week, or he will be so far behind he will sink.
One of my kids has fine motor issues and missing school is a HUGE deal.
Maybe the Op can work with it, though?
Perhaps she can bring her son up to profeciency in the areas missed, and then let him move on.
I find the whole "they have to do everything they missed thing" annoying. My DD missed one day of school and came home with a serious amount of homework. I was not amused. If she missed a week or so, I would talk to the teacher about just doing the basics.
It is almost gauranteed the OP's son will miss more school - with religous holidays, illness, etc. For a child with writing issues there needs to be a better plan than "here are reams of work they must do for the days they missed".
If the Op thinks he will or should have to make up the work when he returns, letting him know this might motivate him to go back to school sooner rather than later. Of course, it may not. Missing today may be more important to him than extra work tomorrow.