Really concerned about lethargic ds

[fartmeistergeneral]

No interest in anything except gaming and tv
Doesn't invite his friends round any more (used to be round every day)
Not remotely interested in doing well at school
No hobbies (used to be interested in music but his instruments just lie gathering dust)
Hangs around with his younger brother and his friends...

what should I do to help???

He's 13

Hmmm, I think there are elements of normal teenage behaviour in there.

My ds was only interested in gaming and tv
Perhaps considering hs age his friendship groups are changing
As I teacher, whilst it's not uncommon for 13 year olds to be turned off school. They do improve as they reach upper school.
Perhaps he has grown out of music hobbies
Hanging round with younger brother and his friends is easier than making new friends.

Some of it sounds like normalish teenage behaviour. Does he seem unhappy? Can you tell us anymore?