Heat grill to high.
Season pork with salt and pepper.
Stir together mustard, fennel seeds, and 1 teaspoon oil; rub mixture all over pork.
Cook couscous in a pot of boiling salted water until al dente, 5 to 7 minutes.
Drain; toss with 1 tablespoon oil.
Drizzle 2 teaspoons oil over vegetables; season with salt and pepper.
Brush grates with oil.
Grill vegetables, flipping once, until tender and charred in places, about 3 minutes for scallions and 6 minutes for squashes.
Grill pork, turning occasionally, until browned on all sides and a thermometer inserted into thickest part registers 138 degrees, 12 to 15 minutes.
Whisk together vinegar, remaining 4 tablespoons oil, and mint.
Season with salt and pepper.
Chop scallions; stir into mint mixture.
Toss couscous with squashes and ½ cup scallion relish.
Slice pork and serve, with couscous and remaining relish.
