Can I buy the Apple Watch and have it attached to his father's account later?
Yes if his father has an iPhone. An iPhone is required to activate Apple Watch. Family Share does limit some features but not many. You may want to make sure you know the difference between the GPS only and Cellular watch models, so you purchase the right watch for the features your grandson would need. I just set up my mother on my iPhone on the Family Share and am very glad she got the Cellular model so she can use the fall detection and/or use SOS to call 911 without a cell phone.
Yes. Out of the box it is not connected to anyone; but, it will need to be paired with an iPhone to work. His watch can be paired with his father's iPhone (or his own) through family share. He will need his own Apple ID.