How to know if a teenager has depression or is self harming?

My son has started isolating himself a lot and doesn't want to do things with anyone anymore. Not even his friends. He just stays in bed most of the day. He also wears long sleeves now which is strange because he is the type to wear shorts in the SNOW! So him all of sudden wearing long sleeves and sweat pants all day is very unusual. Especially since it doesn't get that cold where we live. He doesn't eat dinner with us anymore. Je just takes it to his room and when I was taking out the trash, the trash in his room had his whole dinner in it. He didn't eat at all. He's also been lashing out at his siblings more, telling them to shut up when they are being loud. He's even been snapping at us to. I'm feeling really concerned. He's not a person who is honest about his feelings because my husband teaches all the boys crying is for girls and he will pop them in the mouth if he sees tears, so my son doesn't like talking about his feelings. Feeling scared as a mother.