請教8大師，P大師和各位大俠, 親屬移民問題

My sister and her families get approval(green card), but my brother-in-law doesn't want  to quit his current job, come to US at this time.

My questions are:

1. If he gives up green card, and doesn't come with my sister to US now. Does he have chance to come to US later (like vistiing my sister and his child) ?

2. If not, when his child has US citenzenship ( 6 years later), can his child still apply green card for him ?  Will immigration won't approve his case since he gives up it before?

Thanks you very much!!!