I know this is some weird US thing and some people will tell me that this is normal but: If I wouldn't get paid for each and every hour I work (either in cash at the end of the month or with time to compensate) I wouldn't lift a finger.
Your text doesn't state that he wants you to be there for emergencies. He wants you to be there for when Person A has a Problem in Excel and Person B decides to call you at 2am about an error message she had on her computer for two weeks now.
It will destroy many of your evenings and weekends. You won't be able to go to the bar on Friday night to drink something because you can't take the calls drunk.

So..
Go get a quote from the ISP what it would cost for them to have someone on call on every day. Show that to your boss. (it will be a lot). Tell them you will be on holiday and you will have some time off or be sick one day. MSP will have to cover. Also you won't be doing this for free. Overtime has to be paid or time to compensate.
And if I were you I wouldn't be willing to do this every day. I'd do this maybe for 5 Days a month at max. I'm not living to work - I work so I can be living a good life.