When to involve hospice?

Hello to all, forgive me if this has been answered somewhere in the forum already. I just truly do not have the time to spend researching between work and caring for my grandfather. About two months ago he was diagnosed with Stage 4 adenocarcinoma and has a 12cm tumor on his mediastinum. He just received his 3rd chemo treatment (carbo/alimta) on Thursday. He is not eating anything and is drinking very little. He has no strength to do anything, sleeps all day and is so weak he can hardly speak sometimes. I know a lot of this is due to the recent chemo treatment. However, I'm wondering when should we think about involving hospice? Between my mother and Aunt and I switching off weekly periods of taking care of him he does get plenty of attention. He is scheduled for a PET scan in 1.5 weeks and I was leaning towards waiting to see what it showed before involving hospice. Does anyone have any advice/tips? Thank you all very much.

Best,

John