Foster son doesn't want to go anywhere anymore

Update on the Elmo. My son's did find it! They had to go dumpster diving in the country clubs dumpster but they found it! Anyone who doesn't know what I'm talking about just look up "Welp my boys just lost their whole summer." Short version my 17&18 year old son made fun of their 14 year old foster brother for carrying around an Elmo in his backpack his mom gave to him when he was little. She's no longer alive. My oldest threw it over the fence of the country club and now they both lost their whole summer and are gonna be volunteering.

So my son's after a long hot day at the country club looking for it, they ended up having to go dumpster diving in the dumpster there to find it. Thank God they did because my foster son was distraught. He didn't even let me wash it when they brought it back home. They had an attitude for being out in the heat all day and threw it at him and they lost their phones because of that. My foster son now no longer wants to really leave his room because he's worried they will make fun of him again. Me and my husband still wanted to take him to do something for his 15th birthday but he doesn't want to leave the house because he's scared if he puts it back in his backpack someone will steal it again and if he carries it around others will make fun of him. He can't stay inside all summer break and I don't want him to give up his birthday because my boys were being assholes. I'm probably gonna bring this up to his therapist but mother to mother any advice. He shouldn't spend his birthday or entire summer couped in the house.

Edit: I'm aware at how my kids acted and I'm nipping that in the bud. Personally idk why they started acting like this because believe it or not my kids aren't usually like this. Yes they are angsty teens but we're never mean. They were mainly reserved if anything. Trust me when I say they aren't just getting away with this. Idk if there's some jealousy going on but I am nipping this in the bud.