Can I share Apple Music to someone who has his own family sharing.

It doesn't matter how much he is sharing -- if he cannot be in two Family Sharing groups at one time.

If he is in a Family Sharing group with someone else (and you are not in that Family Sharing group), it doesn't matter how much he is sharing. In order to share Apple Music family with you, he needs to be in your Family Sharing group and he cannot do that without first leaving the other group.

No. He can only be in one Family Sharing group at a time.