Hi there. When you pick the mission from the map, do you see difficulty options like Expert and Master? For your son to complete the campaign and unlock Strand, he must go through the Lightfall story campaign questline, playing through each mission and doing the HyperNet Current strike. Looking at your gaming history with him, he hasn't done the strike yet and you skipped the mission "Headlong", which comes before Desperate Measures. Ask your son to check his Lightfall campaign quest to see what step it's on. Making him the fireteam lead may also help. If he doesn't have the quest, then have him check the abandoned Quests terminal in the Tower next to the Postmaster and pick it up if it's there. If it's not, have him check with Nimbus. After completing the campaign, he'll be directed to speak to Nimbus to complete the quest. Then, go to the Pooka pool to meditate and get Strand.