Just because he showed up at your work crying and asking you to marry him doesn't mean you should or that you have to. Why are you even contemplating getting back with someone who would reconcile with you and then a few days later you find him canoodling with some other chippy while all the time you "forgave" him you actually never did or could?
It's been a year apart, if you think you can make this work and you take him back, don't take him back until you've had couples counceling. (which I see he's agreed to go to with you) You've done nothing in the year apart to educate yourself to forgive him and not bring it up everytime he does something to piss you off. He's done nothing to educate himself about loyalty, integrity, respect. If you marry him and he cheats on you again, it will take you years to get over how stupid you were to reconcile a second time.
It's interesting that you didn't even mention that you love him.
“The willingness to accept responsibility for one’s own life is the source from which self-respect springs.” ~Joan Didion