Could you start with less than 5 minutes? My DS hated writing at that age as well, and I think at the time we aimed for just 1 (short) sentence instead of timing it. Now that he’s 10, 5 minutes isn’t a big deal. I think consistent copywork helped with this. It seemed the more he did, the easier it got for him, even though we started with very little writing.
It’s not something I’ve looked into, but are there exercises to work on small motor skills that would help? Also, instead of writing out spelling sentences could he build the words somehow? We have reading rods and letter tiles that I use with my 5yo to build phonics words. I’ve seen on Pinterest where moms have written letters on legos to build words. Our letter tiles are just printed letters on cardstock, so that would be something you could easily print out.