HI,
At this point you are at the mercy of his Carrier service.
As KiltedTim said whilst there is a Nominal 140 character limit which if sent as one text splurt then it is much more likely to arrive in one piece and therefore in order.
Obviously there are many SMS apps for computers of all different types that are accessing the SMS system overall.
It's down to the individual carriers about how precise they are about the limit and how they split any large text item.
On the iPhone > Settings > Messages > Character count is a setting that can be On or Off.
Mine has always been Off and I have no idea if it influences the SMS sending on the iPhone (or just counts) and whether it effects the Mac sends if it does limit the SMS Messages.

10:01 pm Saturday; June 11, 2016
iMac 2.5Ghz i5 2011 (El Capitan)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
iPhone and an iPad (2)