Yes, you can put molassas in some water. Most horses love it. Just put a dollop in the water and mix it around.
I use it at endurance rides to get horses to drink unfamiliar water and it works great!
Just don't make it permenant!!! To much molassas is not good! A bucket or two of molassas water is enough until he gets used to the water especially if the grass is very moist!
And like desertgal said check he's not dehydrated and is peeing ok! If not then call a vet!