How to RescueEdit
In the boys bedroom, switch to Roofus and use the Digging Hole at the side of the bed in the pot plant to reach the inside of the mini conservatory. Avoid the Grondas and find William at the very end.
You can make the Grondas fight each other if you hop around the glass in between them. You have to be looking away when they try to strike you, though, and if done correctly, you will see both of them fighting, oblivious to your presence. You can grab this chance to rescue (who is apparently getting hurt by the Grondas amidst their fight) while they are distracted.