Like the guys have said, the main difference between the M1 and M1A1 is the 120mm gun. Also the bustle rack was added to all straight M1s when they were all upgraded to M1IPs. Another sure sign are the turret sponson boxes. The M1 and M1IP had short sponson boxes that did not go to the end of the turret. The M1A1 has longer sponson boxes that go almost all the way back to the end of the turret. Additionally, the turret blast panels on the M1/M1IP were 3 assymetrical panels, the M1A1 has two identical panels. The left hull sponson box was replaced by the NBC system, this caused the fire extinguisher port to be slightly relocated.
The last side skirt is not really a good determiner of an M1 or M1A1 since the original ones were just cut out by the unit welder. It was a good idea that eventually made it into production.
Lot of little problems to adjust as well. Overall color was forest green, I know Model Master makes this shade in both spray and bottle (I use it a lot!).
Best bet is to build his kit OOB in the color he wants or try to find the rare original issue Tamiya M1 or find the really cheap Esci/AMT/Ertl version of the M1. We used to spend big money on these two kits to update them to M1A1 standards. Believe me, I still have a full resin upgrade set!