Two of our 4 children live abroad and their children are/ or will be/ bilingual. In one case, trilingual.
Posters who have stressed that one parent speaks only in English to the child at home are spot on.
The parent whose main language is English is the best person to develop their child's English. The other parent should speak to the child in the language of the country in which they live. However, it also works well if English becomes the home language used by both parents but the child, if he/she goes to nursery at 2 or 3, will pick up the other language very easily.
As a language teacher, I am convinced that if young children hear other languages being consistently used around them, they will develop a facility to understand and speak other languages easily. We acquire language passively at first.
Grandparents can help tremendously by speaking their own language to the child, singing to them ( nursery rhymes are great)sending books, etc. But also having the children visit them on their own when they are old enough to do so. Friends with school-age GC from various countries sign them up for day camps here in England when the GC visit during school holidays. This allows them to develop their English with peers in a fun environment.
We brought up our 4 children bilingually from birth, introducing a 3rd language a bit later, always orally. The girls spoke early but our son did not speak until aged 2. Sometimes, bi or multi lingualism can have this effect because the child is processing all these "codes".
Juney64 , enjoy your grandson's bilingualism and participate in it. Of course, learn Spanish if you wish for your own pleasure, but only speak to him in English. It is the best gift you can give him! 