How do I get my dad to play with his grandson early in the morning?

How early is early ? If your son awakens before you want to be up, use age-appropriate behavior techniques like ignoring his early AM waking rather than expecting your Dad to get up & reinforce your son's behavior. If you need your Dad to care-take your son while you get ready for work, ask him to do so directly & politely. If he declines, ask him if anything would change his mind, but accept his answer as final.
Complicated issue. First feed grampa well, entertain him with the delightful antics of his grandsons and conversation with his son & daughter-in-law. Then make sure gramps is off to bed at a decent hour so he can be fresh in the morning with the little guy. Good luck!