It depends on how old he is, where you are going and if it is legal.
No. John Quincy was the SON of John Adams, not the grandson. The only grandson of a former President to later take office was Benjamin Harrison.
No
The possessive form is grandson's.
Your first cousin's grandson son is your grandson's third cousin. Your second cousin's grandson is your grandson's fourth cousin. Etc., etc.
The grandson of your first cousin and your grandson are third cousins to eachother.
Your grandson and your brother's grandson are second cousins to each other.
If you really need a relationship term, use granddaughter-in-law. Most people would merely say "my grandson's wife."
he is the grandson
Your grandson.
Grandson is wyr.
the: le grandson: petit-fils the grandson: le petit-fils
There are many such people that fit the description. You need to be more specific.