hes 12 and im his cousin (21Male), he is an only child to a single mother 

He has severe anxiety with other underlying mental illness, he is medicated however his symptoms seem to be getting worse. 

to put it simply: his anxiety controls him and every situation he ends up being in, its pretty much like a ticking time bomb,

 when we get together as a family, he cant handle all the people and the noise and he makes everyone feel so uncomfortable by begging his mom to leave, cursing her out, and getting angry and lashing out on everyone who tries to help

, he doesn't eat with us since he only eats few foods, his conversations always turn violent or inappropriate, he used to be aggressive but has grown out of that its more verbal now,

 he doesn't have many friends and doesn't go to school in person, or have interest in sports or social gatherings, so pretty much his family is all he has and I was like that myself growing up, but its getting hard for my family to even be around him he just gives everyone such a hard time, he just controls every situation, every family gathering and it seems we all step on egg shells around him and it just gets like weird and awkward 

I wish I could give him solid, sound advice without making him feel like a nuisance to the family, because the thing is we want him around and to just sit with us peacefully and respectfully but im afraid of making him feel bad

id love to give him some man to man advice the problem is that he doesn't want to listen to anyone ever, especially about more intimate topics like this 

what I wish I could say is: stop letting youre anxiety control you, ive felt the same way you have in the past, in fact we aren't currently on some of the same medications! but ive learned how to handle set aside my anger or fear from  noises and people or social gatherings so that I could be a part of a friend group and have a family that wants to be around me just as much as I want to be around him

I wish I could tell him his anxiety looks like anger and being rude or mean even though I know its just him being scared or fearful of whatever the case may be 

he's missing out on life and its making me sad because he's so young 

how could I word this to a kid? to help him feel wanted but to understand that his anxiety is past the point of control and its hard to be around him anymore and its preventing him from living his life? 

how do you say that to a child? 

thanks in advance :)