How To Boost His Self Esteem

My son is 15 and I've really been trying to help boost his self esteem. My husband died a few years back from a heart... Ever since my son has developed a few issues. He REFUSES to sleep in his own room because he's scared something will happen to me and he won't be awake to help me. He started wetting the bed after his dad died so that didn't help with self esteem. He pretty much sleeps in the bed with me. After about 2 months of his peeing in the bed, I bought him protection to wear at night. He doesn't really talk to anyone and doesn't have friends. I put him in therapy and started seeing some improvement. He started recently sleeping in his own bed and only coming to mine when he has a nightmare. The bedwetting still continues but the pull ups help with that. But he still doesn't feel good about himself. My friends has kids around his age and they invited him to a sleepover, but he refused to go because of his problem. He also makes jokes that put himself down and I tell him to stop. With the pandemic he can't see his therapist so he's been a lot lately. I tried to get him to say high to my friends kids( they were on the sidewalks we were on the porch) but he just gets too shy and hides in his room. Can someone please tell me how to make him feel better about himself. I feel if I can help with his self esteem it will help make friends.