I know there is a post about this in a way but Im having alittle bit of a different issue in a way I think lol. when my son goes to his room he never leave it to go to the bathroom, he just goes in his room, pee and poo!. He is great when his is running around and when he is/ or was at school he is great, he has his undies on and he just goes when he needs too. He wets the bed as well at night so he is wearing pull ups at night and he still pees those but not a lot. We do have a 3 year old daughter and she was diagnosedwith austim last year in Dec. Its been so rough and very stressful. I have no idea if he is feeling neglected, I dont even know if he knows what that means. I include him in everything we do that has to do with her therapy. Any adivce is welcome.
Have you asked your son if there's a reason he won't use the bathroom at home? My mom told me when I was younger (age 3) I didn't like using our bathroom because the toilet made such a loud noise. Since your son is using the potty other places, that is likely not the cause, but it's worth asking. Could you place a training toilet in his room as a beginner step to working toward the bathroom?
Sounds like your whole family has had a stressful year -- is your daughter also in potty-training mode? Perhaps that would cause your son to revert back to pre-training habits.
Definitely reach out to your doctor about this at your son's next appointment. If you don't have one soon, it may be worth scheduling a visit.