You can download the BAA app and track your runner. That way you will know where they are on the course so you can plan to be at a certain place as they pass by.
https://www.baa.org/races/boston-marathon/baa-mobile-app
Here are some more hints on the web site
https://www.baa.org/races/boston-marathon/watch/spectators
You should be able to use the T (Boston's subway system) to get around so you can see your runner at multiple places.
I've never spectated at marathon but have run several. I really needed the support the most around miles 20-22--so my advice is to make sure you are there for your runner somewhere around there.
But definitely check with your runner because their needs might be different!
Will you be able to stay late into the day on Monday? Some of the runners won't start until later in the day-I think some of the last finishers might be crossing the finish line as late as 7 pm.
Have fun! That will be such an amazing experience!