He is so lucky to have you bright. Seriously, I hope this family appreciates how much time and effort you are/will be putting in with this child.
I feel as though it would be early to say anything about developmental delays to grandma if it's only been one week but on the other hand it also depends on whether or not there has been discussion about it already between to the two of you etc. so I guess it would really be up to you. He is still pretty fresh and I feel like maybe one more week before having a serious conversation about it might be better?
Safe to say that it seems his vocabulary can use some work and I don't think its too early to talk about that and the importance of him using sentences and asking politely etc, especially in preparation for school as well as toilet training.